面试题6：二叉树转单链表

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:
    void flatten(TreeNode* root) {
        if (root == nullptr)
            return;
        flatten(root->left);
        flatten(root->right);
        TreeNode* tmp = root->right;
        root->right = root->left;
        root->left = nullptr;
        TreeNode* p = root;
        while (p->right)
            p = p->right;
        p->right = tmp;
    }
}