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

click to show hints.

刚开始想偷懒想把几个条件混着写，但是写了半天没写出来。后来把4种分开写，很快就完成通过了。注意不是由树变链。

/**
 * 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 flatten(TreeNode *root) {
        flat(root);
    }
    TreeNode * flat(TreeNode *root)
    {
        TreeNode *tail = root;
        TreeNode * l = root;
        TreeNode * r = root;
        if(root == NULL)return NULL;
        if(root->left == NULL && root->right == NULL)return root;
        if(root->left != NULL && root->right == NULL)
        {
            l= flat(root->left);
            root->right = root->left;
            root->left = NULL;
            return l;
        }
        else if(root->right!= NULL&&root->left == NULL)
        {
            r = flat(root->right);
            return r;
        }
        else
        {
            r = flat(root->right);
            l= flat(root->left);
            l->right = root->right;
            root->right= root->left;
            root->left =NULL;
            return r;
        }
    }
};