12931 - Finding pairs - (1)

#include <stdio.h>
// math way
int arr[100005];
long long int cnt[10005];
int main(){
    int N;
    long long int total = 0;
    scanf("%d", &N);
    for(int i = 0; i < N; i++){
        scanf("%d", &arr[i]);
        cnt[arr[i]]++;
        //printf("cnt[%d] = %d\n", arr[i], cnt[arr[i]]);
    }
    for(int i = 0; i < 10001; i++){
        if(cnt[i] <= 1) continue;
        else {
            // if there are more than one, e.g 5 of the same number
            // it is the same as (5 2) 5 C 2 which is 5 * 4 / 2
            total += (cnt[i]* (cnt[i] - 1)) /2;
        }
    }
    printf("%lld\n", total);
    return 0;
}