Trim a Binary Search Tree（C++修剪二叉搜索树）

解题思路：

（1）递归，如果根节点小于下界，只用修剪右子树

（2）如果根节点大于上界，只用修剪左子树

/**
 * 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:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if(root==NULL) return root;
        if(root->val<low) return trimBST(root->right, low, high);
        if(root->val>high) return trimBST(root->left, low, high);
        
        root->left=trimBST(root->left, low, high);
        root->right=trimBST(root->right, low, high);
        
        return root;
    }
};