Convert Sorted Array to Binary Search Tree

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

balanced tree 是指 tree的 min depth 和max depth不超过某个数值（1）。而不是完全二叉树!!!!

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        return buildTree(num, 0, num.length - 1);
    }
    public TreeNode buildTree(int[] num, int s, int e){
        if(s > e) return null;
        int mid = (s + e) / 2;
        TreeNode root = new TreeNode(num[mid]);
        root.left = buildTree(num, s, mid - 1);
        root.right = buildTree(num, mid + 1, e);
        return root;
    }
}