Recover Binary Search Tree

Problem:

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?

Solution:
/**
 * 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 {
public:
    void recoverTree(TreeNode* root) {
        TreeNode *cur_min = new TreeNode(INT_MIN), *one = NULL, *sec = NULL;
        check(root, cur_min, one, sec);
        if (one && sec) {
            int tmp = one -> val;
            one -> val = sec -> val;
            sec -> val = tmp;
        }
        return;
    }
    void check(TreeNode *root, TreeNode *&cur_min, TreeNode *&one, TreeNode *&sec) {
        if (!root) {
            return;
        }
        check(root -> left, cur_min, one, sec);
        if (cur_min -> val >= root -> val) {
            if (one == NULL) {
                one = cur_min;
            }
            sec = root;
        }
        cur_min = root;
        check(root -> right, cur_min, one, sec);
        return;
    }
};