lintcode——排序列表转换为二分查找树（链表，二叉排序树）

 1 public:
 2     /**
 3      * @param head: The first node of linked list.
 4      * @return: a tree node
 5      */
 6      
 7      void banary_tree(ListNode* start,ListNode* end,TreeNode** root,int n)
 8      {
 9          if(start==end)
10          {
11              *root=new TreeNode(start->val);
12              return;
13          }
14          if(start->next==end)
15          {
16              *root=new TreeNode(end->val);
17              (*root)->left=new TreeNode(start->val);
18              return;
19          }
20          
21          ListNode* p1=start;
22          int step=n/2-1;
23          while(step--)
24          p1=p1->next;
25          
26          *root=new TreeNode(p1->next->val);
27          banary_tree(start,p1,&(*root)->left,n/2);
28          banary_tree(p1->next->next,end,&(*root)->right,n-n/2-1);
29      }
30      
31     TreeNode *sortedListToBST(ListNode *head) {
32         // write your code here
33         if(head==NULL)
34         return NULL;
35         TreeNode *ptr;
36         int len=1;
37         ListNode* ptr1=head;
38         while(ptr1->next!=NULL)
39         {
40             ptr1=ptr1->next;
41             len++;
42         }
43         banary_tree(head,ptr1,&ptr,len);
44         return ptr;
45     }
46 };