[LeetCode]Convert Sorted List to Binary Search Tree

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 /**
10  * Definition for a binary tree node.
11  * struct TreeNode {
12  *     int val;
13  *     TreeNode *left;
14  *     TreeNode *right;
15  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
16  * };
17  */
18 class Solution {
19 public:
20     TreeNode* sortedListToBST(ListNode* head) {
21         return sortedListToBST(head, len(head));
22     }
23 private:
24     TreeNode *sortedListToBST(ListNode *head, int len) {
25         if (len == 0) return nullptr;
26         if (len == 1) return new TreeNode(head->val);
27         
28         int mid = len / 2;
29         TreeNode *root = new TreeNode((afterNthNode(head, mid))->val);
30         root->left = sortedListToBST(head, mid);
31         root->right = sortedListToBST(afterNthNode(head, mid + 1), len - mid - 1);
32         
33         return root;
34     }
35     
36     int len(ListNode *head) {
37         int n = 0;
38         while (head) {
39             head = head->next;
40             ++n;
41         }
42         
43         return n;
44     }
45     
46     ListNode *afterNthNode(ListNode *head, int n) {
47         while (n--) {
48             head = head->next;
49         }
50         
51         return head;
52     }
53 };