[LeetCode] #23 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:
    struct CompareListElement {
        bool operator ()(const ListNode* a, const ListNode* b) const{
            return (a->val > b->val);
        }
    };
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        vector<ListNode*> vecK;
        ListNode* dummyHead = new ListNode(0);
        ListNode* tail = dummyHead;
        for (vector<ListNode *>::size_type i = 0; i < lists.size(); ++i) {
            if (lists[i] != NULL) {
                vecK.push_back(lists[i]);
            }
        }
        std::make_heap(vecK.begin(), vecK.end(), CompareListElement());
        while (!vecK.empty()) {
            tail->next = vecK.front();
            tail = tail->next;
            std::pop_heap(vecK.begin(), vecK.end(), CompareListElement()); 
            vecK.pop_back();
            if (tail->next != NULL) {
                vecK.push_back(tail->next);
                std::push_heap(vecK.begin(), vecK.end(), CompareListElement()); 
            }
        }
        return dummyHead->next;
    }
};