LeetCode OJ--Merge Two Sorted Lists

http://oj.leetcode.com/problems/merge-two-sorted-lists/

有序链表的归并排序

#include <iostream>
using namespace std;

struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
       // default as asend sorted,merge sort
       ListNode *ans = NULL,*ansEnd = NULL,*n1 = l1,*n2 = l2;
       bool firstone = 1;
       while(n1&&n2)
       {
           while(n1->val <= n2 ->val)
           {
               if(firstone == 1)
               {
                   ans = ansEnd = n1;
                   //ansEnd = ans->next;
                   firstone = 0;
                   n1 = n1->next;
               }
               else
               {
                   ansEnd->next = n1;
                   ansEnd = n1;
                   n1 = n1->next;
               }
               if(n1 == NULL)
                   break;
           }
           if(n1 == NULL)
               break;
           while(n1->val > n2->val)
           {
               if(firstone == 1)
               {
                   ans = ansEnd = n2;
                   firstone = 0;
                   n2 = n2->next;
               }
               else
               {
                   ansEnd->next = n2;
                   ansEnd = n2;
                   n2 = n2->next;
               }
               if(n2 == NULL)
                   break;
           }
       }
       if(n1==NULL && n2!= NULL)
       {
           if(firstone ==1)
           {
               ans = n2;
               return ans;
           }
           ansEnd->next = n2;
       }
       else if(n2 == NULL && n1 != NULL)
       {
           if(firstone == 1)
           {
               ans = n1;
               return ans;
           }
           ansEnd->next = n1;
       }
       return ans;
    }
};

int main()
{
    ListNode *n1 = new ListNode(1);
    ListNode *n2 = new ListNode(3);
    ListNode *n3 = new ListNode(5);

    n1->next = n2;

    Solution myS;
    ListNode *ans = myS.mergeTwoLists(NULL,NULL);
    return 0;
}