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Here is an implementation for N-way merge using priority queue data structure. Code requires n sorted vector<int> as input and generates one huge vector containing the result of merge of input vectors. The worst case time complexity of this algorithm is O(m * log(m)) where m is the total number of elements in all the lists.
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| // MergeK | |
| // | |
| // Created by Anirvana Mishra on 6/18/13. | |
| // Copyright (c) 2013 Anirvana Mishra. All rights reserved. | |
| // | |
| #include <iostream> | |
| #include <vector> | |
| #include <queue> | |
| using namespace std; | |
| struct node{ | |
| int value; | |
| int array; | |
| }; | |
| node getaNode(int value, int array){ | |
| node *aNode=new node(); | |
| aNode->value=value; | |
| aNode->array=array; | |
| return *aNode; | |
| } | |
| class compareValue { | |
| public: | |
| bool operator()(node& n1, node& n2) | |
| { | |
| return n1.value>n2.value; | |
| } | |
| }; | |
| vector<int> mergeK(vector<int> lists[], int n){ | |
| priority_queue<node, vector<node>, compareValue> pq; | |
| vector<int> result; | |
| vector<int> pointers(n,0); | |
| node least; | |
| for(int i=0;i<n;i++){ | |
| pq.push(getaNode(lists[i][0], i)); | |
| } | |
| while(!pq.empty()){ | |
| least=pq.top(); | |
| result.push_back(least.value); | |
| pq.pop(); | |
| if(pointers[least.array]>=lists[least.array].size()-1) continue; | |
| pointers[least.array]++; | |
| pq.push(getaNode(lists[least.array][pointers[least.array]], least.array)); | |
| } | |
| return result; | |
| } | |
| int main(int argc, const char * argv[]) //Driver | |
| { | |
| vector<int> lists[3]; | |
| lists[0].push_back(5); | |
| lists[0].push_back(7); | |
| lists[0].push_back(19); | |
| lists[1].push_back(10); | |
| lists[1].push_back(11); | |
| lists[2].push_back(0); | |
| lists[2].push_back(5); | |
| lists[2].push_back(15); | |
| vector<int> result = mergeK(lists, 3); | |
| for(int i=0;i<result.size();i++){ | |
| cout<<result[i]<<" "; | |
| } | |
| return 0; | |
| } | |
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