Find duplicate in an array of N+1 Integers in C++

October 11, 2022

Duplicate in an array of N+1 integers in C++

Here, in this page we will discuss the program to find the duplicate in an array of N+1 integers in C++ . We are Given an array of integers Arr containing N+1 integers where each integer is in the range [1, N] inclusive.

Algorithm :

Take the size of the array from the user and store it on variable say n.
Declare an array of size N and take N elements from the user.
Declare a map which holds the frequency of the elements present in the array.
Now, iterate over the array and store the frequency of elements in the array in the map.
Now, iterate the map and print those keys which have value greater than 1.

Code in C++

#include <bits/stdc++.h>
using namespace std;

int main ()
{

  int n;
  cin >> n;

  int arr[n];

  for (int i = 0; i < n; i++) 
  cin >> arr[i];

  map < int, int >mp;

  for (int i = 0; i < n; i++) 
     mp[arr[i]]++; 

  for (auto it = mp.begin (); it != mp.end (); it++) 
    { 
          if (it->second > 1)
	  cout << it->first << " ";
    }
  return 0;
}

Output

Output :

5

1 1 2 5 5

1 5

Efficient Algorithm to find duplicate in an array of N+1 integers in C++ :

Traverse the given array from 0 to n.
For every element in the array increment the (arr[i]-1)%n‘th element by n.
Now traverse the array again and print all those indices i for which (arr[i]-1)/n is greater than 2. Which guarantees that the number n has been added to that index.

#include <iostream>
using namespace std;

// function to find repeating elements
void printRepeating(int arr[], int n)
{
  for (int i = 0; i < n; i++)
  {
     int index = (arr[i]-1) % n;
     arr[index] += n;
  }

  for (int i = 0; i < n; i++) 
    { 
       if (((arr[i]-1) / n) >= 2)
       cout << i+1 << " "; 
    } 
} 
// Driver code 
  int main() 
 { 
  int n; 
  cin>>n;

  int arr[n];

  for(int i=0; i<n; i++) cin>>arr[i];
  cout << "The repeating elements are: \n";

  // Function call
  printRepeating(arr, n);
  return 0;
}

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