Star

LeetCode 108. Convert Sorted Array to Binary Search Tree

Posted on | In |

Question

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

Explanation

递归，每次找到根节点就行。

Code

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums == null || nums.length == 0) return null;
        return helper(nums, 0, nums.length-1);
    }
    public TreeNode helper(int[] nums, int start, int end) {
        if (nums == null) return null;
        //base case
        if (start > end) return null;
        int mid = start + (end - start)/2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = helper(nums, start, mid-1);
        root.right = helper(nums, mid+1, end);
        return root;
    }
}