LeetCode Trim a Binary Search Tree

原题链接在这里：https://leetcode.com/problems/trim-a-binary-search-tree/description/

题目：

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：

在[L, R]区间外trim. 

Recursive call, stop condition, root == null, return root.

如果root.val 小于 L, 则root右侧有可能在区间内，返回trim后的 root.right. 

如果root.val 大于 R, 则root左侧有可能在区间内，返回trim后的root.left.

如果root.val在区间内, 则对左右两侧分别trim后return root.

Time Complexity: O(n), n 是node数.

Space: O(logn), stack space.

AC Java:

/**
 * 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 root;
        }
        
        if(root.val < L){
            return trimBST(root.right, L, R);
        }
        if(root.val > R){
            return trimBST(root.left, L, R);
        }
        
        root.left = trimBST(root.left, L, R);
        root.right = trimBST(root.right, L, R);
        
        return root;
    }
}