Leetcode: Convert Sorted Array to Binary Search Tree

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

基本上一次过，要注意边界条件的问题：如果在recursion里有两个参数int begin, end, 递归写作recursion(num, mid+1, end), 因为+号的原因，递归中是会出现begin > end 的情况的，所以考虑初始条件的时候应该要考虑充分。

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