Leetcode 669. Trim a Binary Search Tree

题目链接

https://leetcode.com/problems/trim-a-binary-search-tree/description/

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.

Example 1:

Input: 
    1
   / \
  0   2

  L = 1
  R = 2

Output: 
    1
      \
       2

Example 2:

Input: 
    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

题解

采用递归的方式来求解。如果当前节点满足条件，就递归修剪左右子树；当不满足条件时，因为是二叉搜索树，所以左右子树中，一定有一个是在要求的范围内的，把在范围内的节点当做当前节点，继续递归遍历。

代码

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