108. 将有序数组转换为二叉搜索树

 1 public class ConvertSortedArrayToBinarySerachTree108 {
 2     static class TreeNode {
 3         int val;
 4         TreeNode left;
 5         TreeNode right;
 6         TreeNode(int x) {
 7             this.val  = x;
 8         }
 9     }
10     
11     public TreeNode sortedArrayToBST(int[] nums) {
12         return toBST(nums, 0, nums.length - 1);
13     } 
14     
15     public TreeNode toBST(int[] nums, int start, int end) {
16         if(start > end) { //相等的时候需要返回这一个节点，所以不能返回null
17             return null;
18         }
19         //或这当start == end时，直接返回节点
20 /*        if(start == end) {
21             return new TreeNode(nums[start]);
22         }*/
23         int mid = (start + end) / 2;
24         TreeNode root = new TreeNode(nums[mid]);
25         root.left = toBST(nums, start, mid - 1);
26         root.right = toBST(nums, mid + 1, end);
27         return root;
28     }
29 }