776

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> splitBST(TreeNode* root, int V) {
        return dfs(root, V);
    }

    vector<TreeNode*> dfs(TreeNode* root, int V){
        vector<TreeNode*> res(2, NULL);
        if(!root) return res;
        if(root->val > V){
            res[1] = root;
            auto tmp = dfs(root->left, V);
            root->left = tmp[1];
            res[0] = tmp[0];
        }
        else{
            res[0] = root;
            auto tmp = dfs(root->right, V);
            root->right = tmp[0];
            res[1] = tmp[1];
        }
        return res;
    }
};