Arr_to_BST.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if(nums==null || nums.length==0){
            return null;
        }
        
        return bst(nums,0,nums.length-1);
        
    }
    private TreeNode bst(int nums[], int start, int end){

        if(start>end){
            return null;
        }
        int mid=(start+end)/2;
        TreeNode curr = new TreeNode(nums[mid]);
    curr.left=bst(nums,start,mid-1);
    curr.right=bst(nums,mid+1,end);
    return curr;
    
    }

}