Question

I came up with this code but it requires a global variable Rank. Is there any way I can solve this problem without having to have a global variable?

int Rank = 0; 
public int inOrderTraversal(TreeNode node, int n){
    if(node==null)
        return 0;
    int x=inOrderTraversal(node.left,n);
    if(x!=0)return x;
    Rank++;
    if(n==Rank) return node.data;
    int y=inOrderTraversal(node.right,n);
    int c= x==0 ? y:x;
    return c;
}

I am just trying to return the nth term in an in-order traversal of a binary tree.

Answer 1

You can pass a TraversalState object down the recursion invocation chain, and store the number of nodes that you visited in a variable there:

class TraversalState {
    public int rank = 0;
}
...
public int inOrderTraversal(TreeNode node, int n, TraversalState ts){
    if(node==null)
        return 0;
    int x=inOrderTraversal(node.left,n, ts);
    ts.rank++;
    if(n==ts.rank) return node.data;
    int y=inOrderTraversal(node.right,n, ts);
    int c= x==0 ? y:x;
    return c;
}

Now your implementation is thread-safe, because it does not use “global” objects. Invoke it as follows:

int r = inOrderTraversal(myNode, targetN, new TraversalState());