Leetcode -- Merge k Sorted Lists

 
/**  
  * Definition for singly-linked list.  
  * struct ListNode {  
  *   int val;  
  *   ListNode *next;  
  *   ListNode(int x) : val(x), next(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   ListNode *mergeKLists(vector<ListNode *> &lists) {  
     int n = lists.size();  
     if(n <= 0) {  
       return NULL;  
     }  
     ListNode * start = lists[0];  
     for(int i = 1; i < n; ++i) {  
       start = mergeTwoList(start, lists[i]);  
     }  
     return start;  
   }  
   ListNode * mergeTwoList(ListNode * node1, ListNode * node2) {  
     ListNode * dummyHead = new ListNode(-1);  
     ListNode * p = dummyHead;  
     while(node1 != NULL && node2 != NULL) {  
       if(node1->val < node2->val) {  
         p->next = node1;  
         node1 = node1->next;  
       } else {  
         p->next = node2;  
         node2 = node2->next;  
       }  
       p = p->next;  
     }  
     if(node1 != NULL) {  
       p->next = node1;  
     }  
     if(node2 != NULL) {  
       p->next = node2;  
     }  
     ListNode * head = dummyHead->next;  
     dummyHead->next = NULL;  
     delete dummyHead;  
     return head;  
   }  
 };