Leetcode: Recover Binary Search Tree in C++

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

Recover the tree without changing its structure.

Solution:

void preorder(TreeNode *root, TreeNode*& pre, TreeNode*& t1, TreeNode*& t2)
    {
        if(!root)
        return;
        if(root->left)
        preorder(root->left, pre, t1, t2);
        if(pre&&pre->val>root->val)
        {
            t2 = root;
            if(!t1)
            t1 = pre;
        }
        pre = root;
        if(root->right)
        preorder(root->right, pre, t1, t2);
    }
    void recoverTree(TreeNode *root) {
        if(!root)
         return;
        TreeNode* tmp1 = NULL;
        TreeNode* tmp2 = NULL;
        TreeNode* pre = NULL;
        preorder(root, pre, tmp1, tmp2); 
        if(tmp1&&tmp2)
        swap(tmp1->val, tmp2->val);
    }