【LeetCode】 108. Convert Sorted Array to Binary Search Tree 将有序数组转换为二叉搜索树（Easy）（JAVA）

题目地址: https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

题目描述：

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:

题目大意

将一个按照升序排列的有序数组，转换为一棵高度平衡二叉搜索树。

本题中，一个高度平衡二叉树是指一个二叉树每个节点的左右两个子树的高度差的绝对值不超过 1。

解题方法

1、二叉搜索树的前序遍历是一个升序有序数组
2、取中间值，左边的为左子树，右边的为右子树，不断迭代

/**
 * 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) {
        return sH(nums, 0, nums.length - 1);
    }

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

执行用时 : 0 ms, 在所有 Java 提交中击败了 100.00% 的用户
内存消耗 : 39.1 MB, 在所有 Java 提交中击败了 8.70% 的用户