【leetcode】Merge k Sorted Lists(TLE)

先贴TLE的代码。每次遍历取出最小值

/**
 * @author          johnsondu
 * @problem         Merge k Sorted Lists
 * @url             https://leetcode.com/problems/merge-k-sorted-lists/
 * @timeComlexity   O(nm) where n is maximum list length, and m is number of lists. 
 * @spaceComplexity O(n)
 * @strategy        List Insert
 * @status          Time Limited Error
 * @time            22:41, Oct 9 2015
 */

/**
 * 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) {
        ListNode *head = new ListNode(0);
        ListNode *root = head;
        while(1) {
            bool flag = false;
            for(int i = 0; i < lists.size(); i ++) 
                if(lists[i] != NULL) flag = true; 
            if(!flag) break;
            
            flag = false;
            int cur;
            int index = 0;
            for(int i = 0; i < lists.size(); i ++)
            {
                if(lists[i]) {
                    if(!flag) {
                        flag = true;
                        cur = lists[i]->val;
                        index = i;
                    }
                    else {
                        if(cur > lists[i]->val) {
                            cur = lists[i]->val;
                            index = i;
                        }
                    }
                }
            }
            ListNode *tmp = new ListNode(cur);
            head->next = tmp;
            head = head->next;
            lists[index] = lists[index]->next;
        }
        return root->next;
    }
};