Leetcode -- Recover Binary Search Tree

 /**  
  * 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 recoverTree(TreeNode *root) {  
     TreeNode * n1 = NULL;  
     TreeNode * n2 = NULL;  
     TreeNode * prev = NULL;  
     recoverTree(root, n1, n2, prev);  
     if(n1 && n2) {  
     int temp = n1->val;  
     n1->val = n2->val;  
     n2->val = temp;  
     }  
   }  
   void recoverTree(TreeNode * root, TreeNode *& n1, TreeNode *& n2, TreeNode *& prev) {  
     if(!root) {  
       return;  
     }  
      recoverTree(root->left, n1, n2, prev );  
     if(prev && prev->val > root->val) {  
       n2 = root;  
       if(!n1) {  
         n1 = prev;  
       }  
     }  
     prev = root;  
     recoverTree(root->right, n1, n2, prev );  
   }  
 };