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 FindTwo(TreeNode *node, int &temp1, int &temp2, TreeNode* &prenode, TreeNode* &node1, TreeNode* &node2)
{
if(node == NULL)return;
FindTwo(node->left, temp1, temp2, prenode, node1, node2);

if(prenode == NULL)
{
temp1 = node->val;
temp2 = node->val;
}
else
{
temp1 = temp2;
temp2 = node->val;
if(temp1 > temp2)
{
if(node1 == NULL)node1 = prenode;
node2 = node;
}
}
prenode = node;

FindTwo(node->right, temp1, temp2, prenode, node1, node2);
}

void recoverTree(TreeNode *root) {
if(root == NULL) return;
int temp1;
int temp2;
TreeNode *preNode = NULL;
TreeNode *node1 = NULL;
TreeNode *node2 = NULL;
FindTwo(root, temp1, temp2, preNode, node1, node2);
int temp = node1->val;
node1->val = node2->val;
node2->val = temp;
return;
}
};