Balanced binary search tree from doubly linkedlist

Question

I am trying to write this function that takes in a doublyLinkedList and constructs a balanced binary search tree in place. The TreeNode.left is equivalent to the previous pointer and TreeNode.right is like the next pointer. I am taking inspiration from the program here but that doesnt work:

http://www.geeksforgeeks.org/in-place-conversion-of-sorted-dll-to-balanced-bst/

private static TreeNode constructBST2(TreeNode head, int m, int n) {
    TreeNode temp = null;
    if (m < n) {
        int mid = m + (n - m)/ 2;
        TreeNode left = constructBST2(head, m, mid);
        temp = head;
        temp.left = left;
        head = head.right;
        temp.right = constructBST2(head, mid + 1, n);
    }
    return temp;
}

Use http://cs.stackexchange.com/, it is more specific to these type of questions. — Aman Gupta, May 04 '16 at 06:56

score 0 · Answer 1 · answered May 04 '16 at 06:17

Let me try:

private static TreeNode constructBST2(TreeNode root, int r, int m, int n) {
    if (m < n) {
        int leftTreeMid = m + (int)Math.ceil((r - m) / 2);
        int delta = r - leftTreeMid;
        TreeNode left = root;
        for (int i = 0; i < delta; i++)
            left = left.left;
        root.left = left;
        constructBST2(left, leftTreeMid, m, r - 1);

        int rightTreeMid = r + (int)Math.ceil((n - r) / 2);
        delta = rightTreeMid - r;
        TreeNode right = root;
        for(int i = 0; i < delta; i++)
            right = right.right;
        root.right = right;
        constuctBST2(right, rightTreeMid, r + 1, n);
    }
    return root;
}

I haven't tried it at all, maybe you can try it and tell me if it works.

Actually, I'm sure the bottom most nodes (leaf nodes) of the tree would still point their left/right nodes wrongly. But if this sample code makes sense to you, then it should not be hard for you to fix that part. — Jai, May 04 '16 at 06:24

Balanced binary search tree from doubly linkedlist

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