递归

class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if(root==nullptr)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;
    }
};