Merge sort template (with reverse order pair)

#include<cstdio>
#define REP(i, a, b) for(int i = (a); i < (b); i++)
using namespace std;

const int MAXN = 112;
int a[MAXN], n;

void merge_sort(int l, int r) //Here is the left closed and right open interval, the interval does not need to add one and subtract one
{
	if(l + 1 >= r) return;
	int m = (l + r) / 2; // average of l and r
	merge_sort(l, m); merge_sort(m, r); //First recurse, then solve
	
	int p = l, q = m, i = l, t[MAXN];
	while(p < m || q < r)
	{
		if(q >= r || p < m && a[p] <= a[q]) t[i++] = a[p++]; //The right interval is empty or the left interval is not empty and a[p] is better join when
		else t[i++] = a[q++];
	}
	REP(i, l, r) a[i] = t[i];
}

intmain()
{
	scanf("%d", &n);
	REP(i, 0, n) scanf("%d", &a[i]);
	merge_sort (0, n);
	REP(i, 0, n) printf("%d ", a[i]);
	puts("");
	return 0;
}

Inverse pairs satisfy two conditions, i < j and ai > aj

Merge can find the reverse order pair, because it is added in order, so when the right interval is added, the number in the left interval satisfies i < j, and then the number that has not been added on the left must be larger than the current a[q], it should be by size added, so it satisfies ai >aj, so at this time, the counter can add the number of the left interval that has not been added, that is, m-p. Note that the left is closed and the right is open, so m-p does not need to be added by one.

// reverse order version
#include<cstdio>
#define REP(i, a, b) for(int i = (a); i < (b); i++)
using namespace std;

const int MAXN = 112;
int a[MAXN], n, cnt; //If there are multiple sets of data cnt remember to clear 0

void merge_sort(int l, int r)  
{
	if(l + 1 >= r) return;
	int m = (l + r) / 2;          
	merge_sort(l, m); merge_sort(m, r);
	
	int p = l, q = m, i = l, t[MAXN];
	while(p < m || q < r)
	{
		if(q >= r || p < m && a[p] <= a[q]) t[i++] = a[p++];
		else t[i++] = a[q++], cnt += m - p; // just add this sentence
	}
	REP(i, l, r) a[i] = t[i];
}

intmain()
{
	scanf("%d", &n);
	REP(i, 0, n) scanf("%d", &a[i]);
	merge_sort (0, n);
	REP(i, 0, n) printf("%d ", a[i]);
	printf("\n%d\n", cnt);
	return 0;
}