LeetCode: Convert Sorted List to Binary Search Tree
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
地址：https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/
算法：把有序链表转化成平衡的二叉搜索树。首先找到链表的中间节点，然后把中间节点作为根，递归调用前面半个链表作为左子树，递归调用后面半个链表作为右子树。代码：
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 binary tree
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 if(!head){
22 return NULL;
23 }
24 if(!head->next){
25 TreeNode * p = new TreeNode(head->val);
26 return p;
27 }
28 ListNode *pre_first = NULL;
29 ListNode *first_p = head;
30 ListNode *second_p = head->next;
31 while(second_p){
32 pre_first = first_p;
33 first_p = first_p->next;
34 second_p = second_p->next;
35 if(second_p){
36 second_p = second_p->next;
37 }
38 }
39 TreeNode *root = new TreeNode(first_p->val);
40 if(head != first_p){
41 pre_first->next = NULL;
42 root->left = sortedListToBST(head);
43 }
44 root->right = sortedListToBST(first_p->next);
45 return root;
46 }
47 };
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 binary tree
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 if(!head){
22 return NULL;
23 }
24 if(!head->next){
25 TreeNode * p = new TreeNode(head->val);
26 return p;
27 }
28 ListNode *pre_first = NULL;
29 ListNode *first_p = head;
30 ListNode *second_p = head->next;
31 while(second_p){
32 pre_first = first_p;
33 first_p = first_p->next;
34 second_p = second_p->next;
35 if(second_p){
36 second_p = second_p->next;
37 }
38 }
39 TreeNode *root = new TreeNode(first_p->val);
40 if(head != first_p){
41 pre_first->next = NULL;
42 root->left = sortedListToBST(head);
43 }
44 root->right = sortedListToBST(first_p->next);
45 return root;
46 }
47 };
