LeetCode OJ

题目：

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

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

解题思路：

依照前序遍历二叉树的方法进行处理。

代码：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void pre_order(TreeNode * root) {
        if (root == NULL) return;

        if (root->left != NULL)    pre_order(root->left);
        if (root->right != NULL) pre_order(root->right);

        if (root->left != NULL) {
            TreeNode *cur = root->left;
            while (cur->right != NULL) {
                cur = cur->right;
            }
            cur->right = root->right;
            root->right = root->left;
            root->left = NULL;
        }
        return;
    }
    void flatten(TreeNode *root) {
        pre_order(root);
    }
};