LeetCode 669. Trim a Binary Search Tree (Easy)

题目描述：

Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in [L, R] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example1：

Input:     1   / \  0   2  L = 1  R = 2Output:     1      \       2

Example2：

Input:     3   / \  0   4   \    2   /  1  L = 1  R = 3Output:       3     /    2     / 1

题目大意：给一个二叉搜索树，一个闭区间，把搜索树中小于或者大于区间的节点去掉，返回新树的根。

思路：递归，如果左节点值小于区间左侧，那么剪掉左侧，如果右节点值大于区间右侧，剪掉右侧。

c++代码：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode* trimBST(TreeNode* root, int L, int R) {        if (root == nullptr)            return nullptr;        if (root->val < L)            return trimBST(root->right, L, R);        if (root->val > R)            return root = trimBST(root->left, L, R);        else        {            root->left = trimBST(root->left, L, R);            root->right = trimBST(root->right, L, R);            return root;        }    }};