C++ Program To Merge K Sorted Linked Lists Using Min Heap - Set 2 - GeeksforGeeks

10 Jan, 2022
Given k linked lists each of size n and each list is sorted in non-decreasing order, merge them into a single sorted (non-decreasing order) linked list and print the sorted linked list as output.Examples: 
Input: k = 3, n =  4
list1 = 1->3->5->7->NULL
list2 = 2->4->6->8->NULL
list3 = 0->9->10->11->NULL

Output: 0->1->2->3->4->5->6->7->8->9->10->11
Merged lists in a sorted order 
where every element is greater 
than the previous element.

Input: k = 3, n =  3
list1 = 1->3->7->NULL
list2 = 2->4->8->NULL
list3 = 9->10->11->NULL

Output: 1->2->3->4->7->8->9->10->11
Merged lists in a sorted order 
where every element is greater 
than the previous element.
Source: Merge K sorted Linked Lists | Method 2
An efficient solution for the problem has been discussed in Method 3 of this post.
Approach: This solution is based on the MIN HEAP approach used to solve the problem ‘merge k sorted arrays’ which is discussed here. MinHeap: A Min-Heap is a complete binary tree in which the value in each internal node is smaller than or equal to the values in the children of that node. Mapping the elements of a heap into an array is trivial: if a node is stored at index k, then its left child is stored at index 2k + 1 and its right child at index 2k + 2.
Create a min-heap and insert the first element of all the ‘k’ linked lists.As long as the min-heap is not empty, perform the following steps:Remove the top element of the min-heap (which is the current minimum among all the elements in the min-heap) and add it to the result list.If there exists an element (in the same linked list) next to the element popped out in previous step, insert it into the min-heap.Return the head node address of the merged list.
Create a min-heap and insert the first element of all the ‘k’ linked lists.
As long as the min-heap is not empty, perform the following steps:Remove the top element of the min-heap (which is the current minimum among all the elements in the min-heap) and add it to the result list.If there exists an element (in the same linked list) next to the element popped out in previous step, insert it into the min-heap.
Remove the top element of the min-heap (which is the current minimum among all the elements in the min-heap) and add it to the result list.If there exists an element (in the same linked list) next to the element popped out in previous step, insert it into the min-heap.
Remove the top element of the min-heap (which is the current minimum among all the elements in the min-heap) and add it to the result list.
If there exists an element (in the same linked list) next to the element popped out in previous step, insert it into the min-heap.
Return the head node address of the merged list.
Below is the implementation of the above approach:
C++
// C++ implementation to merge k// sorted linked lists// | Using MIN HEAP method#include <bits/stdc++.h>using namespace std;  struct Node {    int data;    struct Node* next;};  // Utility function to create a new nodestruct Node* newNode(int data){    // allocate node    struct Node* new_node = new Node();      // put in the data    new_node->data = data;    new_node->next = NULL;      return new_node;}  // 'compare' function used to build up the// priority queuestruct compare {    bool operator()(        struct Node* a, struct Node* b)    {        return a->data > b->data;    }};  // function to merge k sorted linked listsstruct Node* mergeKSortedLists(    struct Node* arr[], int k){    // priority_queue 'pq' implemented    // as min heap with the    // help of 'compare' function    priority_queue<Node*, vector<Node*>, compare> pq;      // push the head nodes of all    // the k lists in 'pq'    for (int i = 0; i < k; i++)        if (arr[i] != NULL)            pq.push(arr[i]);            // Handles the case when k = 0       // or lists have no elements in them          if (pq.empty())        return NULL;          struct Node *dummy = newNode(0);      struct Node *last = dummy;        // loop till 'pq' is not empty    while (!pq.empty()) {          // get the top element of 'pq'        struct Node* curr = pq.top();        pq.pop();            // add the top element of 'pq'          // to the resultant merged list        last->next = curr;          last = last->next;                    // check if there is a node        // next to the 'top' node        // in the list of which 'top'        // node is a member        if (curr->next != NULL)            // push the next node of top node in 'pq'            pq.push(curr->next);    }      // address of head node of the required merged list    return dummy->next;}  // function to print the singly linked listvoid printList(struct Node* head){    while (head != NULL) {        cout << head->data << " ";        head = head->next;    }}  // Driver program to test aboveint main(){    int k = 3; // Number of linked lists    int n = 4; // Number of elements in each list      // an array of pointers storing the head nodes    // of the linked lists    Node* arr[k];      // creating k = 3 sorted lists    arr[0] = newNode(1);    arr[0]->next = newNode(3);    arr[0]->next->next = newNode(5);    arr[0]->next->next->next = newNode(7);      arr[1] = newNode(2);    arr[1]->next = newNode(4);    arr[1]->next->next = newNode(6);    arr[1]->next->next->next = newNode(8);      arr[2] = newNode(0);    arr[2]->next = newNode(9);    arr[2]->next->next = newNode(10);    arr[2]->next->next->next = newNode(11);      // merge the k sorted lists    struct Node* head = mergeKSortedLists(arr, k);      // print the merged list    printList(head);      return 0;}
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Complexity Analysis: 
Time Complexity: O(N * log k) or O(n * k * log k), where, ‘N’ is the total number of elements among all the linked lists, ‘k’ is the total number of lists, and ‘n’ is the size of each linked list.Insertion and deletion operation will be performed in min-heap for all N nodes.Insertion and deletion in a min-heap require log k time.
Auxiliary Space: O(k). The priority queue will have atmost ‘k’ number of elements at any point of time, hence the additional space required for our algorithm is O(k).
Please refer complete article on Merge k sorted linked lists | Set 2 (Using Min Heap) for more details!
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