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merge-k-sorted-lists.cpp
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merge-k-sorted-lists.cpp
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/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
struct Compare {
bool operator()(ListNode* const& a, ListNode* const& b) {
return a->val > b->val;
}
};
// Time - O(N LogK), Space - O(K)
// where N = total elements in all lists, K = number of lists
ListNode* solve(vector<ListNode*> &lists) {
if(lists.empty()) {
return nullptr;
}
ListNode *dummy = new ListNode(0);
ListNode *tail = dummy;
priority_queue<ListNode*, vector<ListNode*>, Compare> pq;
for(auto& list: lists) {
if(list != nullptr) {
pq.emplace(list);
}
}
while(!pq.empty()) {
tail->next = pq.top();
pq.pop();
tail = tail->next;
if(tail->next != nullptr) {
pq.emplace(tail->next);
}
}
return dummy->next;
}
ListNode* Solution::mergeKLists(vector<ListNode*> &A) {
return solve(A);
}