文章作者：Tyan
博客：noahsnail.com ?|? CSDN ?|? 簡書

1. 問題描述

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

2. 求解

這個題主要是根據一個有序數組構造二叉查找樹（樹的左結點小于根節點，根節點小于右結點，子樹具有同樣的性質）。構造方法主要是遞歸，每次構建子樹時都需要將數組分成左右兩半，左邊的構建左子樹，右邊的構建右子樹，中間元素構造根節點。

/**
 * 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) {
        return buildBinarySearchTree(nums, 0, nums.length - 1);
    }
    
    public TreeNode buildBinarySearchTree(int[] nums, int start, int end) {
        if(start > end) {
            return null;
        }
        int mid = (start + end) / 2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = buildBinarySearchTree(nums, start, mid - 1);
        root.right = buildBinarySearchTree(nums, mid + 1, end);
        return root;
    }
}