【leetcode】Flatten Binary Tree to Linked List

 1 class Solution {
 2 public:
 3     queue<TreeNode*> Q;
 4 
 5     void preorder(TreeNode *root)
 6     {
 7         if(root)
 8         {
 9           Q.push(root);
10           preorder(root->left);
11           preorder(root->right);
12         }
13           
14     }
15 
16     void flat(TreeNode * root)
17     {
18 
19         if(!Q.empty())
20         {
21           root->right=Q.front();
22           root->left=NULL;
23           Q.pop();
24           flat(root->right);
25         }
26 
27         
28     }
29 
30     void flatten(TreeNode* root) {
31        if(root==NULL) return ;
32         preorder(root);
33         root=Q.front();
34         Q.pop();
35         root->left=NULL;
36         flat(root);
37 
38     }
39 };