【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

 

 

class Solution {
public:
    queue<TreeNode*> Q;

    void preorder(TreeNode *root)
    {
        if(root)
        {
          Q.push(root);
          preorder(root->left);
          preorder(root->right);
        }
          
    }

    void flat(TreeNode * root)
    {

        if(!Q.empty())
        {
          root->right=Q.front();
          root->left=NULL;
          Q.pop();
          flat(root->right);
        }

        
    }

    void flatten(TreeNode* root) {
       if(root==NULL) return ;
        preorder(root);
        root=Q.front();
        Q.pop();
        root->left=NULL;
        flat(root);

    }
};