let 108. Convert Sorted Array to Binary Search Tree

/**
* 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;

return build(nums,0,nums.length-1);
}

public TreeNode build(int []nums,int start,int end) {

if(start>end) return null;
int rootIndex=start+(end-start)/2;

TreeNode root=new TreeNode(nums[rootIndex]);
root.left=build(nums,start,rootIndex-1);
root.right=build(nums,rootIndex+1,end);
return root;
}
}