Middle-题目53：114. Flatten Binary Tree to Linked List

题目原文：
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

题目大意：
把一个二叉树平铺到右子树上。
题目分析：
先递归平铺左右子树，再把左子树接到根节点右孩子上即可。
源码：（language：java）

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

        TreeNode ptr = root.left;
        if(ptr!=null){
            while(ptr.right!=null){
                ptr = ptr.right;
            }
            ptr.right = root.right;
            root.right = root.left;
            root.left = null;
        }
    }
}

成绩：
1ms,beats 34.42%,众数1ms,65.58%