114. 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

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

可以看到这个链表是前序遍历的结果，这里可以进行逆过程。先向右遍历，再向左遍历，弹栈的结果保存为prev，那么root的left变为null，root的right变为prev。代码如下：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    TreeNode prev = null;        public void flatten(TreeNode root) {        if (root == null) {            return;        }        flatten(root.right);        flatten(root.left);        root.left = null;        root.right = prev;        prev = root;    }}