class Solution {
public TreeNode sortedArrayToBST(int[] num) {
if(num.length==0){
return null;
}
TreeNode head = helper(num,0,num.length-1);
return head;
}
public TreeNode helper(int[] num, int low , int high){
if(low>high){
return null;

    }
    int mid = low+(high-low)/2;
    TreeNode node = new TreeNode(num[mid]);
    node.left = helper(num,low,mid-1);
    node.right = helper(num,mid+1,high);
    return node;
}
}