LeetCode.114 Flatten Binary Tree to LinkedList

题目：

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.

分析：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private TreeNode pre=null;
    public void flatten(TreeNode root) {
        //递归实现将左右子树变成一个单串
        if(root==null) return ;
        flatten(root.right);
        flatten(root.left);
        root.right=pre;
        root.left=null;
        pre=root;
    }
}