LeetCode刷题笔录Convert Sorted Array to Balanced Binary Search Tree

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

这题可以一眼看出是要用递归，不会是叫你写一个BST的插入。每次选出sorted array中间的那个元素作为当前的node,这个node的左儿子是左半边数组的中间元素，右儿子是右半边数组的中间元素。这样就有Subproblem了。

coding的时候注意subproblem的范围是(low, mid - 1)和(mid + 1, high)，因为mid元素已经用过了。

发现自己递归还是要多练练，想到了这是递归也用了很久才写出代码，sigh

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedArrayToBST(int[] num) {        return helper(num, 0, num.length - 1);    }    public TreeNode helper(int[] num, int low, int high){        if(low > high)            return null;        int mid = (low + high) / 2;                TreeNode node = new TreeNode(num[mid]);        node.left = helper(num, low, mid - 1);        node.right = helper(num, mid + 1, high);        return node;                }}