/**
 * 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)
            return null;
        int len = num.length;
        return build(num, 0, len - 1);
    }
    
    private TreeNode build(int[] num, int lo, int hi) {
        if (lo > hi)
            return null;
        int mid = lo + (hi - lo) / 2;
        TreeNode node = new TreeNode(num[mid]);
        node.left = build(num, lo, mid - 1);
        node.right = build(num, mid + 1, hi);
        return node;
    }
}