Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.
Example:
Given the sorted array: [-10,-3,0,5,9],
One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:
0
/ -3 9
/ /
-10 5
注意edge case: start > end的时候返回null
time: O(n), space: O(n) -- O(logn) for recursive call, and O(n) for constructing BST
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public TreeNode sortedArrayToBST(int[] nums) { if(nums == null || nums.length == 0) return null; return helper(nums, 0, nums.length - 1); } private TreeNode helper(int[] nums, int start, int end) { 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; } }
108. Convert Sorted Array to Binary Search Tree - Easy
原文：https://www.cnblogs.com/fatttcat/p/10145175.html