Leetcode-Convert Sorted Array to BST

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public TreeNode sortedArrayToBST(int[] num) {
12         if (num.length==0)
13             return null;
14         
15         int end = num.length-1;
16         TreeNode root = sortedArrayToBSTRecur(num,0,end);
17         return root;
18     }
19     
20     public TreeNode sortedArrayToBSTRecur(int[] num, int head, int end){
21         if (head==end){
22             TreeNode root = new TreeNode(num[head]);
23             return root;
24         }
25         
26         if (head+1==end){
27             TreeNode root = new TreeNode(num[end]);
28             TreeNode child = new TreeNode(num[head]);
29             root.left = child;
30             return root;
31         }
32         
33         //Calculate the median index.
34         int len = end-head;
35         int mid = head + len/2 + len%2;
36         TreeNode root = new TreeNode(num[mid]);
37         TreeNode leftChild = sortedArrayToBSTRecur(num,head,mid-1);
38         TreeNode rightChild = sortedArrayToBSTRecur(num,mid+1,end);
39         root.left = leftChild;
40         root.right = rightChild;
41         return root;
42     }
43 }