[leetcode] merge k sort list

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    typedef struct{        bool operator()(ListNode* node_1, ListNode* node_2){            return node_1->val < node_2->val;        }    }Comp;    ListNode *mergeKLists(vector<ListNode *> &lists) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int k=0 ;        if (lists.size()==0)            return NULL;        if (lists.size() == 1)            return lists[0];                    ListNode* head = NULL;        ListNode* pre = NULL;        multiset<ListNode*, Comp> temp_heap;                for(int i = 0; i< lists.size();i++){            if ( lists[i] != NULL){                temp_heap.insert(lists[i]);                k++;                lists[i] = lists[i]->next;            }        }                while(k>1){            ListNode* temp = *temp_heap.begin();            if (head == NULL){                head = temp;            }            else                pre->next = temp;            pre = temp;            temp_heap.erase(temp_heap.begin());            if (temp->next != NULL)                temp_heap.insert(temp->next);            else                k--;                    }                if(pre != NULL)pre->next =*temp_heap.begin();elsehead = *temp_heap.begin();        return head;    }};

1. 依然用multiset实现heap

2. 

if(pre != NULL)pre->next =*temp_heap.begin();elsehead = *temp_heap.begin();

最开始没有做判断，导致如果只有一个list为非空是，出错