Write a C program to find the duplicate element in an array?

Find the duplicate number in array: For example, an array with length 5 contains numbers {1, 2, 3, 4, 4}, the duplicated number is 4. Suppose that the duplicated number in the array is m in given example. The sum of all numbers in the array, denoted as arrSum, should be the result of 1+2+...+(n-1)+m. It is not difficult to get the sum of 1+2+...+(n-1), which is denoted as sum. The duplicated number m will be the difference of arrSum and sum. Following C program demonstrates how do we detect a repeated element in an integer array?.

/* An array of size N contains numbers ranging from 1 to N-1.
   There is exactly one number is duplicated in the array.
   Write a C program to find the duplicated number?
*/
#include <stdio.h>
#define N 5
int main()
{
  int arr[N]; //contains integers from 1 to N-1
  int arrSum = 0; //sum of array elements
  int sum = (N * (N - 1)) / 2; //sum of 1 to N-1
  int i; //loop counter
  printf("Enter %d integers form 1 to %d, with one duplicate:", N, N-1);
  for (i = 0; i < N; i++)
  {
    scanf ("%d", &arr[i]);
    arrSum += arr[i];
  }
 
  printf("\nDuplicate number is: %d\n", arrSum - sum);
}


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