Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

Pre-order Traversal

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (root == null) return;
        Stack stack = new Stack();
        stack.push(root);
        TreeNode lastNode = null;
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (lastNode != null) {
                lastNode.left = null;
                lastNode.right = node;
            }
            lastNode = node;
            TreeNode left = node.left;
            TreeNode right = node.right;
            if (right != null) {
                stack.push(right);
            }
            if (left != null) {
                stack.push(left);
            }
        }
    }
}