108. Convert Sorted Array to Binary Search Tree

//O(n) time and space
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if(nums==null || nums.length==0) return null;
        return helper(nums,0,nums.length-1);
    }
    public TreeNode helper(int[] nums,int start, int end){
        if(start>end) return null;
        int mid = (start+end)/2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = helper(nums,start,mid-1);
        root.right = helper(nums,mid+1,end);
        return root;
    }
}