FlattenBinaryTreeToLinkedList.java

/*
 * Flatten a binary tree to a fake "linked list" 
 * in pre-order traversal.

Here we use the right pointer in TreeNode as the 
next pointer in ListNode.

Example
              1
               \
     1          2
    / \          \
   2   5    =>    3
  / \   \          \
 3   4   6          4
                     \
                      5
                       \
                        6
Note
Don't forget to mark the left child of each node 
to null. Or you will get Time Limit Exceeded or 
Memory Limit Exceeded.

Challenge
Do it in-place without any extra memory.
 */
/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class FlattenBinaryTreeToLinkedList {

    /**
     * @param root: a TreeNode, the root of the binary tree
     * @return: nothing
     */
    public void flatten(TreeNode root) {
        while (root != null) {
            if (root.left != null) {
                TreeNode cur = root.left;
                while (cur.right != null) {
                    cur = cur.right;
                }
                cur.right = root.right;
                root.right = root.left;
                root.left = null;
            }
            root = root.right;
        }
    }

/*******************************************************************/

    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
            node.left = null;
            if (!stack.isEmpty()) {
                node.right = stack.peek();
            }
        }
    }

/*******************************************************************/

    public void flatten(TreeNode root) {
        flatten(root, null);
    }

    public TreeNode flatten(TreeNode root, TreeNode result) {
        if (root == null) {
            return result;
        }
        result = flatten(root.right, result);
        root.right = flatten(root.left, result);
        root.left = null;
        return root;
    }

/*******************************************************************/

    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        flatten(root.left);
        flatten(root.right);
        TreeNode right = root.right;
        root.right = root.left;
        root.left = null;
        while (root.right != null) {
            root = root.right;
        }
        root.right = right;
    }

}