Convert Sorted Array to Binary Search Tree With Minimal Height.java

/**
*Given a sorted (increasing order) array, Convert it to create a binary tree with minimal height.
*Given [1,2,3,4,5,6,7], return

     4
   /   \
  2     6
 / \    / \
1   3  5   7

time = O(n)
space = O(logn)
*/

/**
 * 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) {
        if (nums == null || nums.length == 0) {
            return null;
        }
        TreeNode result = helper(nums, 0, nums.length - 1);
        return result;
    }
    
    private TreeNode helper(int[] nums, int left, int right) {
        if (left > right) {
            return null;
        }
        int mid = left + (right - left) / 2;
        TreeNode node = new TreeNode(nums[mid]);
        node.left = helper(nums, left, mid - 1);
        node.right = helper(nums, mid + 1, right);
        return node;
    }
}