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?

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {private:    void recoverTreeHelper(TreeNode* root, TreeNode* &preNode, TreeNode* &node1, TreeNode* &node2) {                if (root == NULL)            return;                    recoverTreeHelper(root->left, preNode, node1, node2);                if (preNode != NULL)        {            if (root->val < preNode->val)            {                node2 = root;                                if (node1 == NULL)                {                    node1 = preNode;                }            }        }        preNode = root;        recoverTreeHelper(root->right, preNode, node1, node2);            }    public:    void recoverTree(TreeNode* root) {                TreeNode *preNode = NULL;        TreeNode *node1 = NULL;        TreeNode *node2 = NULL;                 recoverTreeHelper(root, preNode, node1, node2);                if (node1 != NULL and node2 != NULL)        {            int tmp = node1->val;            node1->val = node2->val;            node2->val = tmp;        }    }};