inversions.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#define MAXSIZ 100010
long long int sortandcount(long long int a[],long long int low,long long int high);
long long int mergeandcountsplit(long long int b[],long long int lowi,long long int midi,long long int highi);
int main(int argc, char const *argv[])
{
	long long int array[MAXSIZ]/*input array*/, n/*array size*/;
	long long int inversions,i;
	scanf("%lld", &n);
	for (i = 0; i < n; ++i)
		scanf("%lld", &array[i]);/*reading*/
	inversions=sortandcount(array,0,n-1);
	printf("%lld\n", inversions);
	return 0;
}
long long int sortandcount(long long int a[],long long int low,long long int high)
{
	long long int x=0,y=0,z=0,mid=(low+high)/2;
	if (low<high)
	{
		x=sortandcount(a,low, mid);
		y=sortandcount(a,mid+1,high);
		z=mergeandcountsplit(a,low,mid,high);
	}
	return x+y+z;
}
long long int mergeandcountsplit(long long int b[],long long int lowi,long long int midi,long long int highi)
{
	long long int stepmother[MAXSIZ];/*step mother for our sorting array*/
									/*It's very pity that, every time step 
									mother grows his child in a sorted way 
									from an unsorted  and then every time 
									the natural mother reclaims the sorted son*/
									/*too pity*/
	long long int i=lowi,j=midi+1,k=0;
	long long int inversion_S=0;
	while(i<=midi&&j<=highi)
	{
		if (b[i]<=b[j])
		{
			stepmother[k++]=b[i++];
		}
		else
		{
			inversion_S+=midi-i+1;/*Birthday Party*/
			stepmother[k++]=b[j++];
		}
	}
	while(i<=midi)
		stepmother[k++]=b[i++];
	while(j<=highi)
		stepmother[k++]=b[j++];
	while(k--)
		b[lowi+k]=stepmother[k];
	return inversion_S;
}