Given a binary tree, flatten it to a linked list in-place.

For example,
Given

1
2
3
4
5
    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
2
3
4
5
6
7
8
9
10
11
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

二叉树转链表

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/**
 * 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 helper(TreeNode* &root, TreeNode* &last) {
        last = root;
        if (root == nullptr)
            return ;
        
        TreeNode *mylast, *temp = root->right;
        helper(root->left, mylast);
        if (mylast) {
            last = mylast;
            last->right = root->right;
            root->right = root->left;
            root->left = nullptr;
        }
        
        helper(root->right, mylast);
        if (mylast)
            last = mylast;
        
    }
    
    void flatten(TreeNode* root) {
        TreeNode *last;
        helper(root, last);
    }
};