669. Trim a Binary Search Tree

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

题意

给定一棵二叉搜索树和最小最大边界L和R，修剪这棵树，使得其中的元素位于L和R之间。

思路

当root的值位于L和R之间，则递归修剪其左右子树，返回root。
当root的值小于L，则其左子树的值都小于L，抛弃左子树，返回修剪过的右子树。
当root的值大于R，则其右子树的值都大于R，抛弃右子树，返回修剪过的左子树

/**
 * 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;
        }
        if(root.val<L) return trimBST(root.right,  L, R);
        return trimBST(root.left, L, R);//the left case
    }
}