代码随想录：修剪二叉搜索树

/**
 * 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) {
        // 修剪一棵树时，若根节点在区间内，应该确认他的左子树是否小于区间，右子树是否大于区间。
        // 若根节点大于区间，只用找他的左子树是否在区间内
        // 若根节点小于区间，只用找他的右子树是否在区间内
        if (root == NULL)
            return root;
        if (root->val >= low && root->val <= high) {
            root->left = trimBST(root->left, low, high);
            root->right = trimBST(root->right, low, high);
        } else if (root->val > high) {
            root = trimBST(root->left, low, high);
        } else if (root->val < low) {
            root = trimBST(root->right, low, high);
        }
        return root;
    }
};