108. Convert Sorted Array to Binary Search Tree

 1 class Solution {
 2     public TreeNode sortedArrayToBST(int[] nums) {
 3         int size = nums.length;
 4         return helper(nums, 0, size - 1);
 5         
 6     }
 7     
 8     public TreeNode helper(int[] nums, int lo, int hi) {
 9         if(lo > hi) return null;
10         int mid = lo + (hi - lo) / 2;
11         TreeNode root = new TreeNode(nums[mid]);
12         root.left = helper(nums, lo , mid-1);
13         root.right = helper(nums, mid+1, hi);
14         return root;
15     }
16 }