669题

方法：后序遍历
搜索二叉树定义：左子树小于头节点，头节点小于右节点（在做题目时忘记了该定义）

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