LeetCode-Merge k Sorted Lists

哎，这两天都是在做水题，做水题都还一点状态都木有，

经常一道题调好久~~~

哎，好拙计！

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (lists.empty()) {
            return NULL;
        }
        ListNode *res = NULL;
        ListNode *pre;
        ListNode *cur;
        vector<ListNode *> prelist = lists;
        bool first = false;
        while (!prelist.empty()) {
            bool val = false;
            vector<ListNode *> curlist;
            for (size_t i = 0; i < prelist.size(); ++i) {
                if (prelist[i] == NULL) {
                    continue;
                }
                if (!val) {
                    val = true;
                    cur = prelist[i];
                    continue;
                }
                if (prelist[i]->val < cur->val) {
                    curlist.push_back(cur);
                    cur = prelist[i];
                }
                else {
                    curlist.push_back(prelist[i]);
                }
            }
            if(!val) {
                if (res == NULL) {
                    return NULL;
                }
                break;
            }
            curlist.push_back(cur->next);
            prelist = curlist;
            if (!first) {
                first = true;
                res = cur;
                pre = cur;
            }
            else {
                pre->next = cur;
                pre = cur;
            }
        }
        if (res == NULL) {
            return res;
        }
        cur->next = NULL;
        return res;
    }
};