[LeetCode]Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Note:
A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

/**
 * 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 treeWalk(TreeNode *root, TreeNode *&prv, TreeNode *&first, TreeNode *&second)
    {
      if(root)
      {
          treeWalk(root->left,prv,first,second);
          if((prv!=NULL)&&(prv->val>root->val))
          {
              if(first==NULL) first=prv;
              second=root;
          }
          prv=root;
          treeWalk(root->right,prv,first,second);
      }
    }
    void recoverTree(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode* first=NULL;
        TreeNode* second=NULL;
        TreeNode* prv=NULL;
        treeWalk(root,prv,first,second);
        swap(first->val,second->val);
    }
};