[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);

    }

};