leetcode-114. 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

思路：递归处理，引用二叉链表的思想，使用pre记录上一个分支的指针。

Accepted Code:

/**
 * 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 {
private:
    TreeNode* pre=nullptr;
public:
    void flatten(TreeNode* root) {
        if(root==nullptr)
        return;
        flatten(root->right);
        flatten(root->left);
        root->right=pre;
        root->left=nullptr;
        pre=root;
    }
};