Question

It’s not easy to find red-black tree implementations on the net, especially for learning.

Where can I find a simple red-black tree implementation (C# preferred)?

Answer 1

Generic red-black trees aren’t “simple” by default.
But if you put a small restriction on them and make them “left-leaning”, then they become simpler.
Take a look at this MSDN blog post.

I’ve copy-pasted (with slight modifications) the code from that post here (in C#):

using System;
using System.Collections.Generic;
using System.Diagnostics;

/// <summary>Implements a left-leaning red-black tree.</summary>
/// <remarks>
/// Based on the research paper "Left-leaning Red-Black Trees"
/// by Robert Sedgewick. More information available at:
/// http://www.cs.princeton.edu/~rs/talks/LLRB/RedBlack.pdf
/// http://www.cs.princeton.edu/~rs/talks/LLRB/08Penn.pdf
/// </remarks>
/// <typeparam name="TKey">Type of keys.</typeparam>
/// <typeparam name="TValue">Type of values.</typeparam>
public class LeftLeaningRedBlackTree<TKey, TValue>
{
    /// <summary>Stores the key comparison function.</summary>
    private Comparison<TKey> _keyComparison;

    /// <summary>Stores the value comparison function.</summary>
    private Comparison<TValue> _valueComparison;

    /// <summary>Stores the root node of the tree.</summary>
    private Node _rootNode;

    /// <summary>Represents a node of the tree.</summary>
    /// <remarks>Using fields instead of properties drops execution time by about 40%.</remarks>
    [DebuggerDisplay("Key={Key}, Value={Value}, Siblings={Siblings}")]
    private class Node
    {
        /// <summary>Gets or sets the node's key.</summary>
        public TKey Key;

        /// <summary>Gets or sets the node's value.</summary>
        public TValue Value;

        /// <summary>Gets or sets the left node.</summary>
        public Node Left;

        /// <summary>Gets or sets the right node.</summary>
        public Node Right;

        /// <summary>Gets or sets the color of the node.</summary>
        public bool IsBlack;

        /// <summary>Gets or sets the number of "siblings" (nodes with the same key/value).</summary>
        public int Siblings;
    }

    /// <summary>Initializes a new instance of the LeftLeaningRedBlackTree class implementing a normal dictionary.</summary>
    /// <param name="keyComparison">The key comparison function.</param>
    public LeftLeaningRedBlackTree(Comparison<TKey> keyComparison)
    {
        if (null == keyComparison)
        {
            throw new ArgumentNullException("keyComparison");
        }
        _keyComparison = keyComparison;
    }

    /// <summary>Initializes a new instance of the LeftLeaningRedBlackTree class implementing an ordered multi-dictionary.</summary>
    /// <param name="keyComparison">The key comparison function.</param>
    /// <param name="valueComparison">The value comparison function.</param>
    public LeftLeaningRedBlackTree(Comparison<TKey> keyComparison, Comparison<TValue> valueComparison)
        : this(keyComparison)
    {
        if (null == valueComparison)
        {
            throw new ArgumentNullException("valueComparison");
        }
        _valueComparison = valueComparison;
    }

    /// <summary>Gets a value indicating whether the tree is acting as an ordered multi-dictionary.</summary>
    private bool IsMultiDictionary
    {
        get { return null != _valueComparison; }
    }

    /// <summary>Adds a key/value pair to the tree.</summary>
    /// <param name="key">Key to add.</param>
    /// <param name="value">Value to add.</param>
    public void Add(TKey key, TValue value)
    {
        _rootNode = Add(_rootNode, key, value);
        _rootNode.IsBlack = true;
        AssertInvariants();
    }

    /// <summary>Removes a key (and its associated value) from a normal (non-multi) dictionary.</summary>
    /// <param name="key">Key to remove.</param>
    /// <returns>True if key present and removed.</returns>
    public bool Remove(TKey key)
    {
        if (IsMultiDictionary)
        {
            throw new InvalidOperationException("Remove is only supported when acting as a normal (non-multi) dictionary.");
        }
        return Remove(key, default(TValue));
    }

    /// <summary>Removes a key/value pair from the tree.</summary>
    /// <param name="key">Key to remove.</param>
    /// <param name="value">Value to remove.</param>
    /// <returns>True if key/value present and removed.</returns>
    public bool Remove(TKey key, TValue value)
    {
        int initialCount = Count;
        if (null != _rootNode)
        {
            _rootNode = Remove(_rootNode, key, value);
            if (null != _rootNode)
            {
                _rootNode.IsBlack = true;
            }
        }
        AssertInvariants();
        return initialCount != Count;
    }

    /// <summary>Removes all nodes in the tree.</summary>
    public void Clear()
    {
        _rootNode = null;
        Count = 0;
        AssertInvariants();
    }

    /// <summary>Gets a sorted list of keys in the tree.</summary>
    /// <returns>Sorted list of keys.</returns>
    public IEnumerable<TKey> GetKeys()
    {
        TKey lastKey = default(TKey);
        bool lastKeyValid = false;
        return Traverse(
            _rootNode,
            n => !lastKeyValid || !object.Equals(lastKey, n.Key),
            n =>
            {
                lastKey = n.Key;
                lastKeyValid = true;
                return lastKey;
            });
    }

    /// <summary>Gets the value associated with the specified key in a normal (non-multi) dictionary.</summary>
    /// <param name="key">Specified key.</param>
    /// <returns>Value associated with the specified key.</returns>
    public TValue GetValueForKey(TKey key)
    {
        if (IsMultiDictionary)
        {
            throw new InvalidOperationException("GetValueForKey is only supported when acting as a normal (non-multi) dictionary.");
        }
        Node node = GetNodeForKey(key);
        if (null != node)
        {
            return node.Value;
        }
        else
        {
            throw new KeyNotFoundException();
        }
    }

    /// <summary>Gets a sequence of the values associated with the specified key.</summary>
    /// <param name="key">Specified key.</param>
    /// <returns>Sequence of values.</returns>
    public IEnumerable<TValue> GetValuesForKey(TKey key)
    {
        return Traverse(GetNodeForKey(key), n => 0 == _keyComparison(n.Key, key), n => n.Value);
    }

    /// <summary>Gets a sequence of all the values in the tree.</summary>
    /// <returns>Sequence of all values.</returns>
    public IEnumerable<TValue> GetValuesForAllKeys()
    {
        return Traverse(_rootNode, n => true, n => n.Value);
    }

    /// <summary>Gets the count of key/value pairs in the tree.</summary>
    public int Count { get; private set; }

    /// <summary>Gets the minimum key in the tree.</summary>
    public TKey MinimumKey
    {
        get { return GetExtreme(_rootNode, n => n.Left, n => n.Key); }
    }

    /// <summary>Gets the maximum key in the tree.</summary>
    public TKey MaximumKey
    {
        get { return GetExtreme(_rootNode, n => n.Right, n => n.Key); }
    }

    /// <summary>Returns true if the specified node is red.</summary>
    /// <param name="node">Specified node.</param>
    /// <returns>True if specified node is red.</returns>
    private static bool IsRed(Node node)
    {
        if (null == node)
        {
            // "Virtual" leaf nodes are always black
            return false;
        }
        return !node.IsBlack;
    }

    /// <summary>Adds the specified key/value pair below the specified root node.</summary>
    /// <param name="node">Specified node.</param>
    /// <param name="key">Key to add.</param>
    /// <param name="value">Value to add.</param>
    /// <returns>New root node.</returns>
    private Node Add(Node node, TKey key, TValue value)
    {
        if (null == node)
        {
            // Insert new node
            Count++;
            return new Node { Key = key, Value = value };
        }

        if (IsRed(node.Left) && IsRed(node.Right))
        {
            // Split node with two red children
            FlipColor(node);
        }

        // Find right place for new node
        int comparisonResult = KeyAndValueComparison(key, value, node.Key, node.Value);
        if (comparisonResult < 0)
        {
            node.Left = Add(node.Left, key, value);
        }
        else if (0 < comparisonResult)
        {
            node.Right = Add(node.Right, key, value);
        }
        else
        {
            if (IsMultiDictionary)
            {
                // Store the presence of a "duplicate" node
                node.Siblings++;
                Count++;
            }
            else
            {
                // Replace the value of the existing node
                node.Value = value;
            }
        }

        if (IsRed(node.Right))
        {
            // Rotate to prevent red node on right
            node = RotateLeft(node);
        }

        if (IsRed(node.Left) && IsRed(node.Left.Left))
        {
            // Rotate to prevent consecutive red nodes
            node = RotateRight(node);
        }

        return node;
    }

    /// <summary>Removes the specified key/value pair from below the specified node.</summary>
    /// <param name="node">Specified node.</param>
    /// <param name="key">Key to remove.</param>
    /// <param name="value">Value to remove.</param>
    /// <returns>True if key/value present and removed.</returns>
    private Node Remove(Node node, TKey key, TValue value)
    {
        int comparisonResult = KeyAndValueComparison(key, value, node.Key, node.Value);
        if (comparisonResult < 0)
        {
            // * Continue search if left is present
            if (null != node.Left)
            {
                if (!IsRed(node.Left) && !IsRed(node.Left.Left))
                {
                    // Move a red node over
                    node = MoveRedLeft(node);
                }

                // Remove from left
                node.Left = Remove(node.Left, key, value);
            }
        }
        else
        {
            if (IsRed(node.Left))
            {
                // Flip a 3 node or unbalance a 4 node
                node = RotateRight(node);
            }
            if ((0 == KeyAndValueComparison(key, value, node.Key, node.Value)) && (null == node.Right))
            {
                // Remove leaf node
                Debug.Assert(null == node.Left, "About to remove an extra node.");
                Count--;
                if (0 < node.Siblings)
                {
                    // Record the removal of the "duplicate" node
                    Debug.Assert(IsMultiDictionary, "Should not have siblings if tree is not a multi-dictionary.");
                    node.Siblings--;
                    return node;
                }
                else
                {
                    // Leaf node is gone
                    return null;
                }
            }
            // * Continue search if right is present
            if (null != node.Right)
            {
                if (!IsRed(node.Right) && !IsRed(node.Right.Left))
                {
                    // Move a red node over
                    node = MoveRedRight(node);
                }
                if (0 == KeyAndValueComparison(key, value, node.Key, node.Value))
                {
                    // Remove leaf node
                    Count--;
                    if (0 < node.Siblings)
                    {
                        // Record the removal of the "duplicate" node
                        Debug.Assert(IsMultiDictionary, "Should not have siblings if tree is not a multi-dictionary.");
                        node.Siblings--;
                    }
                    else
                    {
                        // Find the smallest node on the right, swap, and remove it
                        Node m = GetExtreme(node.Right, n => n.Left, n => n);
                        node.Key = m.Key;
                        node.Value = m.Value;
                        node.Siblings = m.Siblings;
                        node.Right = DeleteMinimum(node.Right);
                    }
                }
                else
                {
                    // Remove from right
                    node.Right = Remove(node.Right, key, value);
                }
            }
        }

        // Maintain invariants
        return FixUp(node);
    }

    /// <summary>Flip the colors of the specified node and its direct children.</summary>
    /// <param name="node">Specified node.</param>
    private static void FlipColor(Node node)
    {
        node.IsBlack = !node.IsBlack;
        node.Left.IsBlack = !node.Left.IsBlack;
        node.Right.IsBlack = !node.Right.IsBlack;
    }

    /// <summary>Rotate the specified node "left".</summary>
    /// <param name="node">Specified node.</param>
    /// <returns>New root node.</returns>
    private static Node RotateLeft(Node node)
    {
        Node x = node.Right;
        node.Right = x.Left;
        x.Left = node;
        x.IsBlack = node.IsBlack;
        node.IsBlack = false;
        return x;
    }

    /// <summary>Rotate the specified node "right".</summary>
    /// <param name="node">Specified node.</param>
    /// <returns>New root node.</returns>
    private static Node RotateRight(Node node)
    {
        Node x = node.Left;
        node.Left = x.Right;
        x.Right = node;
        x.IsBlack = node.IsBlack;
        node.IsBlack = false;
        return x;
    }

    /// <summary>Moves a red node from the right child to the left child.</summary>
    /// <param name="node">Parent node.</param>
    /// <returns>New root node.</returns>
    private static Node MoveRedLeft(Node node)
    {
        FlipColor(node);
        if (IsRed(node.Right.Left))
        {
            node.Right = RotateRight(node.Right);
            node = RotateLeft(node);
            FlipColor(node);

            // * Avoid creating right-leaning nodes
            if (IsRed(node.Right.Right))
            {
                node.Right = RotateLeft(node.Right);
            }
        }
        return node;
    }

    /// <summary>Moves a red node from the left child to the right child.</summary>
    /// <param name="node">Parent node.</param>
    /// <returns>New root node.</returns>
    private static Node MoveRedRight(Node node)
    {
        FlipColor(node);
        if (IsRed(node.Left.Left))
        {
            node = RotateRight(node);
            FlipColor(node);
        }
        return node;
    }

    /// <summary>Deletes the minimum node under the specified node.</summary>
    /// <param name="node">Specified node.</param>
    /// <returns>New root node.</returns>
    private Node DeleteMinimum(Node node)
    {
        if (null == node.Left)
        {
            // Nothing to do
            return null;
        }

        if (!IsRed(node.Left) && !IsRed(node.Left.Left))
        {
            // Move red node left
            node = MoveRedLeft(node);
        }

        // Recursively delete
        node.Left = DeleteMinimum(node.Left);

        // Maintain invariants
        return FixUp(node);
    }

    /// <summary>Maintains invariants by adjusting the specified nodes children.</summary>
    /// <param name="node">Specified node.</param>
    /// <returns>New root node.</returns>
    private static Node FixUp(Node node)
    {
        if (IsRed(node.Right))
        {
            // Avoid right-leaning node
            node = RotateLeft(node);
        }

        if (IsRed(node.Left) && IsRed(node.Left.Left))
        {
            // Balance 4-node
            node = RotateRight(node);
        }

        if (IsRed(node.Left) && IsRed(node.Right))
        {
            // Push red up
            FlipColor(node);
        }

        // * Avoid leaving behind right-leaning nodes
        if ((null != node.Left) && IsRed(node.Left.Right) && !IsRed(node.Left.Left))
        {
            node.Left = RotateLeft(node.Left);
            if (IsRed(node.Left))
            {
                // Balance 4-node
                node = RotateRight(node);
            }
        }

        return node;
    }

    /// <summary>Gets the (first) node corresponding to the specified key.</summary>
    /// <param name="key">Key to search for.</param>
    /// <returns>Corresponding node or null if none found.</returns>
    private Node GetNodeForKey(TKey key)
    {
        // Initialize
        Node node = _rootNode;
        while (null != node)
        {
            // Compare keys and go left/right
            int comparisonResult = _keyComparison(key, node.Key);
            if (comparisonResult < 0)
            {
                node = node.Left;
            }
            else if (0 < comparisonResult)
            {
                node = node.Right;
            }
            else
            {
                // Match; return node
                return node;
            }
        }

        // No match found
        return null;
    }

    /// <summary>Gets an extreme (ex: minimum/maximum) value.</summary>
    /// <typeparam name="T">Type of value.</typeparam>
    /// <param name="node">Node to start from.</param>
    /// <param name="successor">Successor function.</param>
    /// <param name="selector">Selector function.</param>
    /// <returns>Extreme value.</returns>
    private static T GetExtreme<T>(Node node, Func<Node, Node> successor, Func<Node, T> selector)
    {
        // Initialize
        T extreme = default(T);
        Node current = node;
        while (null != current)
        {
            // Go to extreme
            extreme = selector(current);
            current = successor(current);
        }
        return extreme;
    }

    /// <summary>Traverses a subset of the sequence of nodes in order and selects the specified nodes.</summary>
    /// <typeparam name="T">Type of elements.</typeparam>
    /// <param name="node">Starting node.</param>
    /// <param name="condition">Condition method.</param>
    /// <param name="selector">Selector method.</param>
    /// <returns>Sequence of selected nodes.</returns>
    private IEnumerable<T> Traverse<T>(Node node, Func<Node, bool> condition, Func<Node, T> selector)
    {
        // Create a stack to avoid recursion
        Stack<Node> stack = new Stack<Node>();
        Node current = node;
        while (null != current)
        {
            if (null != current.Left)
            {
                // Save current state and go left
                stack.Push(current);
                current = current.Left;
            }
            else
            {
                do
                {
                    for (int i = 0; i <= current.Siblings; i++)
                    {
                        // Select current node if relevant
                        if (condition(current))
                        {
                            yield return selector(current);
                        }
                    }
                    // Go right - or up if nothing to the right
                    current = current.Right;
                }
                while ((null == current) &&
                       (0 < stack.Count) &&
                       (null != (current = stack.Pop())));
            }
        }
    }

    /// <summary>Compares the specified keys (primary) and values (secondary).</summary>
    /// <param name="leftKey">The left key.</param>
    /// <param name="leftValue">The left value.</param>
    /// <param name="rightKey">The right key.</param>
    /// <param name="rightValue">The right value.</param>
    /// <returns>CompareTo-style results: -1 if left is less, 0 if equal, and 1 if greater than right.</returns>
    private int KeyAndValueComparison(TKey leftKey, TValue leftValue, TKey rightKey, TValue rightValue)
    {
        // Compare keys
        int comparisonResult = _keyComparison(leftKey, rightKey);
        if ((0 == comparisonResult) && (null != _valueComparison))
        {
            // Keys match; compare values
            comparisonResult = _valueComparison(leftValue, rightValue);
        }
        return comparisonResult;
    }

    /// <summary>Asserts that tree invariants are not violated.</summary>
    [Conditional("Debug")]
    private void AssertInvariants()
    {
        // Root is black
        Debug.Assert((null == _rootNode) || _rootNode.IsBlack, "Root is not black");
        // Every path contains the same number of black nodes
        Dictionary<Node, Node> parents = new Dictionary<LeftLeaningRedBlackTree<TKey, TValue>.Node, LeftLeaningRedBlackTree<TKey, TValue>.Node>();
        foreach (Node node in Traverse(_rootNode, n => true, n => n))
        {
            if (null != node.Left)
            {
                parents[node.Left] = node;
            }
            if (null != node.Right)
            {
                parents[node.Right] = node;
            }
        }
        if (null != _rootNode)
        {
            parents[_rootNode] = null;
        }
        int treeCount = -1;
        foreach (Node node in Traverse(_rootNode, n => (null == n.Left) || (null == n.Right), n => n))
        {
            int pathCount = 0;
            Node current = node;
            while (null != current)
            {
                if (current.IsBlack)
                {
                    pathCount++;
                }
                current = parents[current];
            }
            Debug.Assert((-1 == treeCount) || (pathCount == treeCount), "Not all paths have the same number of black nodes.");
            treeCount = pathCount;
        }
        // Verify node properties...
        foreach (Node node in Traverse(_rootNode, n => true, n => n))
        {
            // Left node is less
            if (null != node.Left)
            {
                Debug.Assert(0 > KeyAndValueComparison(node.Left.Key, node.Left.Value, node.Key, node.Value), "Left node is greater than its parent.");
            }
            // Right node is greater
            if (null != node.Right)
            {
                Debug.Assert(0 < KeyAndValueComparison(node.Right.Key, node.Right.Value, node.Key, node.Value), "Right node is less than its parent.");
            }
            // Both children of a red node are black
            Debug.Assert(!IsRed(node) || (!IsRed(node.Left) && !IsRed(node.Right)), "Red node has a red child.");
            // Always left-leaning
            Debug.Assert(!IsRed(node.Right) || IsRed(node.Left), "Node is not left-leaning.");
            // No consecutive reds (subset of previous rule)
            //Debug.Assert(!(IsRed(node) && IsRed(node.Left)));
        }
    }
}