[LeetCode] 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

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void flatten(TreeNode *root) {        while(root){            if(root -> left){                TreeNode *pre = root -> left;                while(pre -> right)                    pre = pre -> right;                pre -> right = root -> right;                root -> right = root -> left;                root -> left = NULL;            }            root = root -> right;        }    }};