114. Flatten Binary Tree to Linked List， pre节点的妙用

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

代码及思路：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void flatten(TreeNode* root) {        TreeNode* pre = NULL;//作为右值之前一定要先赋值。        help(root, pre);    }    void help(TreeNode* &root, TreeNode* &pre){//这里的引用传递，相当于是一个全局变量了        if(root == NULL) return;        help(root->right, pre);        help(root->left, pre);              root->right = pre;        root->left = NULL;        pre = root;    }};