Question

I’ve made red-black-tree implementation based on Cormen, but I must have broke something as it doesn’t work like it should. I believe I rewrote Cormen correctly but I have no idea what is wrong then… How do I know that.. I took 10 values and checked how tree should look (http://secs.ceas.uc.edu/~franco/C321/html/RedBlack/redblack.html) and mine does look different. So I ask kindly for any tips that could help me find out what’s wrong, whole code is pretty long, but I can’t reproduce error without it, sorry about that. I believe guilty are rotate and/or fixup after insertion…

EDIT: New code, but it still causes red and even black violations though I’m could swear I just rewrote pseudocode to C++…

#include <cstdio>
#include <algorithm>
#include <string>


enum rbt_color { RED, BLACK };

struct rbt_node
{
    int key; //klucz
    int sub_tree; //wielkość poddrzewa
    std::string data; //wartość (napis do 21 znaków)
    rbt_node *left; //lewy syn
    rbt_node *right; //prawy syn
    rbt_node *parent;
    rbt_color color; //kolor
};


int is_RED(rbt_node *root)
{
    return root != NULL && root->color == RED;
}

int is_BLACK(rbt_node *root)
{
    return root != NULL && root->color == BLACK;
}

rbt_node *make_node(int key, std::string data)
{
    rbt_node *new_node = new rbt_node; 
    new_node->key = key;
    new_node->data = data;
    new_node->color = RED;
    new_node->left = NULL;
    new_node->right = NULL;
    new_node->sub_tree = 1; //inicjalna wartość
    return new_node;
}

void add_node(rbt_node *&tree, rbt_node *node, rbt_node *parent)
{
        if(tree == NULL)
        {
            node->parent = parent;
            tree = node;
        }
        else if(node->key < tree->key)
        {
            tree->sub_tree += 1;
            add_node(tree->left, node, tree);
        }
        else if(node->key > tree->key)
        {
            tree->sub_tree += 1;
            add_node(tree->right, node, tree);
        }
}


//funkcja testująca drzewo, źródło http://www.eternallyconfuzzled.com/tuts/datastructures/jsw_tut_rbtree.aspx (trochę ulepszyłem)
int rbt_assert (rbt_node *root)
{
   int lh, rh;

   if ( root == NULL )
     return 1;
   else {
     rbt_node *ln = root->left;
     rbt_node *rn = root->right;

     /* Consecutive RED links */
     if ( is_RED ( root ) ) {
       if ( is_RED ( ln ) || is_RED ( rn ) ) {
         puts ( "RED violation" );
         printf("VIOLATION AT KEY: %d\n", root->key);
        //return 0;
       }
     }

     lh = rbt_assert ( ln );
     rh = rbt_assert ( rn );

     if (1 + (ln ? ln->sub_tree : 0) + (rn ? rn->sub_tree : 0) != root->sub_tree) 
     {
          puts ("Subtree violation");
          printf("VIOLATION AT KEY: %d\n", root->key);
          return 0;
     }

     if (root->left != NULL && root->left->parent != root || root->right != NULL && root->right->parent != root) 
     {
          puts ("Parent violation");
          printf("VIOLATION AT KEY: %d\n", root->key);
          return 0;
     }

     /* Invalid binary search tree */
     if ( ( ln != NULL && ln->key >= root->key )
       || ( rn != NULL && rn->key <= root->key ) )
     {
       puts ( "Binary tree violation" );
       return 0;
     }

     /* BLACK height mismatch */
     if ( lh != 0 && rh != 0 && lh != rh ) {
       puts ( "BLACK violation" );
       return 0;
     }

     /* Only count BLACK links */
     if ( lh != 0 && rh != 0 )
       return is_RED ( root ) ? lh : lh + 1;
     else
       return 0;
   }
 }




void left_rotate(rbt_node *&root, rbt_node *&node)
{
    rbt_node *new_node = node->right;
    if(new_node != NULL)
    {
        node->right = new_node->left;
        if(new_node->left != NULL)
            new_node->left->parent = node;

        if(node->parent == NULL)
            root = new_node;
        else if(node == node->parent->left)
            node->parent->left = new_node;
        else
            node->parent->right = new_node;

        new_node->left = node;

        //aktualizujemy rozmiar poddrzewa
        new_node->sub_tree = node->sub_tree;
        node->sub_tree = 1;
        if(node->left != NULL)
            node->sub_tree += node->left->sub_tree;
        if(node->right != NULL)
            node->sub_tree += node->right->sub_tree;

        new_node->parent = node->parent;
        new_node->left->parent = new_node;


    }
}

void right_rotate(rbt_node *&root, rbt_node *& node)
{

    rbt_node *new_node = node->left;
    if(new_node != NULL)
    {
        node->left = new_node->right;
        if(new_node->right != NULL)
            new_node->right->parent = node;



        if(node->parent == NULL)
            root = new_node;
        else if(node == node->parent->right)
            node->parent->right = new_node;
        else
            node->parent->left = new_node;

        new_node->right = node;

            //aktualizujemy rozmiar poddrzewa
        new_node->sub_tree = node->sub_tree;
        node->sub_tree = 1;
        if(node->left != NULL)
            node->sub_tree += node->left->sub_tree;
        if(node->right != NULL)
            node->sub_tree += node->right->sub_tree;

        new_node->parent = node->parent;
        new_node->right->parent = new_node;

    }
}


void add_rbt_node(rbt_node *&root, int key, std::string data, rbt_node *parent)
{
    rbt_node *element = make_node(key, data);
    add_node(root, element, parent);
    while(element != root && element->parent->color == RED)
    {
        if(element->parent == element->parent->parent->left)
        {
            rbt_node *uncle = element->parent->parent->right;
            if(uncle != NULL && uncle->color == RED)
            {
                element->parent->color == BLACK;
                uncle->color = BLACK;
                element->parent->parent->color = RED;
                element = element->parent->parent;
            }
            else
            {
                if(element == element->parent->right)
                {
                    element = element->parent;
                    left_rotate(root, element);
                }
                element->parent->color = BLACK;
                element->parent->parent->color = RED;
                right_rotate(root, element->parent->parent);
            }
        }
        else
        {
            rbt_node *uncle = element->parent->parent->left;
            if(uncle != NULL && uncle->color == RED)
            {
                element->parent->color = BLACK;
                uncle->color = BLACK;
                element->parent->parent->color = RED;
                element = element->parent->parent;
            }
            else
            {
                if(element == element->parent->left)
                {
                    element = element->parent;
                    right_rotate(root, element);
                }
                element->parent->color = BLACK;
                element->parent->parent->color = RED;
                left_rotate(root, element->parent->parent);
            }
        }
    }
    root->color = BLACK;

}

void search_key(rbt_node *root, int key)
{
    if(root == NULL)
        printf("-\n");
    else if(root->key == key)
        printf("%s\n", root->data.c_str());
    else if(root->key < key)
        search_key(root->right, key);
    else if(root->key > key)
        search_key(root->left, key);
}

void min_key(rbt_node *root, int number)
{
    if(root != NULL)
    {
        int rank = 1;
        if(root->left != NULL)
            rank += root->left->sub_tree;
        if(rank == number)
            printf("%s\n", root->data.c_str());
        else if(number < rank)
            min_key(root->left, number);
        else
            min_key(root->right, number - rank);
    }
}

void print_out(rbt_node *root)
{
    if(root != NULL)
    {
        printf("%d %s ", root->key, root->data.c_str());
        if(root->color == BLACK)
            printf("black ");
        else
            printf("red ");
        if(root->parent != NULL)
            printf("%d ",root->parent->key);
        else
            printf("- ");
        if(root->left != NULL)
            printf("%d ",root->left->key);
        else
            printf("- ");
        if(root->right != NULL)
            printf("%d ",root->right->key);
        else
            printf("- ");
        printf("\n");

        print_out(root->left);
        if(root->right != NULL)
        {
            print_out(root->right);
        }
    }
}



int main()
{

    int key;
    char data [21];
    char operation;
    rbt_node *root = NULL;
    while(scanf("%c",&operation) != EOF)
    {
        switch(operation)
        {
            case 'I':
                scanf("%d",&key);
                scanf("%s",data);
                add_rbt_node(root, key, data, NULL);
                break;
            case 'S':
                scanf("%d",&key);
                search_key(root, key);
                break;
            case 'F':
                scanf("%d",&key);
                if(key <= root->sub_tree && key != 0)
                    min_key(root, key);
                else
                    printf("-\n");
                break;
            case 'G':
                printf("%d\n", rbt_assert(root));
                break;
            case 'P':
                //print_out(root);
                break;
        }
    }
}

Answer 1

One problem is that this line (the 12th line of add_rbt_node):

element->parent->color == BLACK;

should almost certainly read:

element->parent->color = BLACK;

And I’ve confirmed that fixing this solves the particular issue you highlighted in your comment. If you had turned on all warnings, your compiler would’ve likely caught this problem for you. It’s a simple and obvious mistake.