leetcode-114. Flatten Binary Tree to Linked List

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

我这里为了不分情况讨论。先把右节点入队，然后左节点移动给右节点，然后当前节点等于其右节点（如果右节点为空则出队）。这样就避免了分类讨论不过即使分类讨论结果复杂度也差不多。可能还要快一些

/** * 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) {        Stack<TreeNode> sta = new Stack<TreeNode>();        TreeNode tmp = root;        while(tmp!=null){            if(tmp.right!=null){                sta.push(tmp.right);                tmp.right =null;            }            if(tmp.left != null){                tmp.right = tmp.left;                tmp.left = null;            }            if(tmp.right==null && !sta.isEmpty()){                tmp.right = sta.pop();            }            tmp = tmp.right;        }    }}