LeetCode99—Recover Binary Search Tree

class Solution {
private:
    void helper(TreeNode*root, TreeNode*&pre, vector<TreeNode*>&res)
    {
        if (root == NULL)
            return;
        helper(root->left, pre, res);
        if (pre != NULL&&pre->val > root->val)
        {
            res.push_back(pre);
            res.push_back(root);
        }
        pre = root;
        helper(root->right, pre, res);
    }

public:
    void recoverTree(TreeNode* root) {
        vector<TreeNode*>res;
        TreeNode*pre = NULL;
        helper(root, pre, res);
        if (res.size() > 0)
        {
            if(res.size()==2)
                swap(res[0]->val, res[1]->val);
            if(res.size()==3)
                swap(res[0]->val,res[2]->val);
            if(res.size()==4)
                swap(res[0]->val,res[3]->val);
        }
    }
};