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
其实这个顺序就是前序遍历的顺序，做孩子都为NULL，右孩子为下一个先序遍历的节点。

void flatten(TreeNode* root) {
         if(!root) return;
         stack<TreeNode*> q;//前序遍历不能用队列啊！
         q.push(root);
         vector<TreeNode*> res;
         while(q.size()>0){
             TreeNode* p=q.top();
             q.pop();
             res.push_back(p);
             if(p->right) q.push(p->right);
             if(p->left) q.push(p->left);
         }
         res.push_back(NULL);
         for(int i=0;i<res.size()-1;i++){
             res[i]->left=res[res.size()-1];
             res[i]->right=res[i+1];
         }
    }