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Merge two lists using merge() function | C++ STL

Example of merge() function in C++ STL: Here, we are going to learn how to merge two lists using merge() function in C++ STL program?
Submitted by IncludeHelp, on October 31, 2018

Problem statement

Given two lists and we have to merge them.

Here, we are implementing two programs 1) to merge two unsorted lists and 2) to merge two sorted lists

Merge two unsorted lists

Here is an example with sample input and output:

Input: 
list1: {20, 10, 40, 30, 50}
list2: {90, 60, 50, 70, 80}
    
Output: 
Merged list:
20 10 40 30 50 90 60 50 70 80

C++ program to merge two unsorted lists

#include <iostream>
#include <list>
using namespace std;

// function to display the list
void dispList(list<int> L) {
  // declaring iterator to the list
  list<int>::iterator l_iter;
  for (l_iter = L.begin(); l_iter != L.end(); l_iter++) cout << *l_iter << " ";
  cout << endl;
}

int main() {
  // declaring a list
  list<int> iList1 = {20, 10, 40, 30, 50};
  list<int> iList2 = {90, 60, 50, 70, 80};

  // printing list elements
  cout << "List1 elements are" << endl;
  dispList(iList1);
  cout << "list2 elements are" << endl;
  dispList(iList2);

  // merging list2 into list1
  iList1.merge(iList2);

  cout << "Merged list elements are" << endl;
  dispList(iList1);

  return 0;
}

Output

List1 elements are
20 10 40 30 50
list2 elements are
90 60 50 70 80
Merged list elements are
20 10 40 30 50 90 60 50 70 80

Merge two sorted lists

Here is an example with sample input and output:

Input:
list1: {20, 10, 40, 30, 50}
list2: {90, 60, 50, 70, 80}

Output:
List1 elements are
20 10 40 30 50
list2 elements are
90 60 50 70 80

//sorting the lists 
List1 (sorted) elements are
10 20 30 40 50
List2 (sorted) elements are
50 60 70 80 90

Merged list elements are
10 20 30 40 50 50 60 70 80 90

C++ program to merge two sorted lists

#include <iostream>
#include <list>
using namespace std;

// function to display the list
void dispList(list<int> L) {
  // declaring iterator to the list
  list<int>::iterator l_iter;
  for (l_iter = L.begin(); l_iter != L.end(); l_iter++) cout << *l_iter << " ";
  cout << endl;
}

int main() {
  // declaring a list
  list<int> iList1 = {20, 10, 40, 30, 50};
  list<int> iList2 = {90, 60, 50, 70, 80};

  // printing list elements
  cout << "List1 elements are" << endl;
  dispList(iList1);
  cout << "list2 elements are" << endl;
  dispList(iList2);

  // sort the lists
  iList1.sort();
  iList2.sort();

  // printing sorted list elements
  cout << "List1 (sorted) elements are" << endl;
  dispList(iList1);
  cout << "List2 (sorted) elements are" << endl;
  dispList(iList2);

  // merging list2 into list1
  iList1.merge(iList2);

  cout << "Merged list elements are" << endl;
  dispList(iList1);

  return 0;
}

Output

List1 elements are
20 10 40 30 50
list2 elements are
90 60 50 70 80
List1 (sorted) elements are
10 20 30 40 50
List2 (sorted) elements are
50 60 70 80 90
Merged list elements are
10 20 30 40 50 50 60 70 80 90