leetcode 023 —— Merge k sorted linked lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

思路，采用小顶堆

/** * 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) {      vector<ListNode*>::iterator it = lists.begin();      while(it != lists.end()) {          if(*it == NULL) lists.erase(it);          else ++it;      }      if(lists.size() < 1) return NULL;        ListNode *head = NULL, *cur = NULL;      make_heap(lists.begin(), lists.end(), comp());        while(lists.size() > 0) {          if(head == NULL) head = cur = lists[0];          else cur = cur->next = lists[0];            pop_heap(lists.begin(), lists.end(), comp());          int last = lists.size() - 1;          if(lists[last]->next != NULL) {              lists[last] = lists[last]->next;              push_heap(lists.begin(), lists.end(), comp());          }          else lists.pop_back();      }        return head;  }   class comp {   public:      bool operator() (const ListNode* l, const ListNode* r) const {          return (l->val > r->val);      }  }; };