// Merge k Sorted Lists
class Solution {
public:
  ListNode *mergeKLists(vector<ListNode *> &lists) {
    struct Cmp {
      bool operator()(const ListNode *x, const ListNode *y) const {
        return x->val > y->val;
      }
    };
    priority_queue<ListNode*, vector<ListNode*>, Cmp> pq;
    for (auto x: lists)
      if (x)
        pq.push(x);
    ListNode *head = NULL, **cur = &head;
    while (! pq.empty()) {
      *cur = pq.top();
      pq.pop();
      ListNode *x = (*cur)->next;
      if (x)
        pq.push(x);
      cur = &((*cur)->next);
    }
    return head;
  }
};