#include<bits/stdc++.h>
using namespace std;
#define S 100001
#define ll int

ll arr[S];
ll ans;

void merge(int l ,int h , ll (&arr)[S])
{
	ll mid=l+(h-l)/2;
	long long ar[S];
	int i=l;
	int j=mid+1;
	int k=0;

	while(i<=mid && j<=h)
	{
		if (arr[i]>arr[j])
		{
			ans+=(mid-i+1);
			ar[k++]=arr[j++];
		}
		else
			ar[k++]=arr[i++];
	}

	while(i<=mid)
		ar[k++]=arr[i++];
		
	while (j<=h)
		ar[k++]=arr[j++];

	for (int i = 0; i < k; ++i)
		arr[l++]=ar[i];
	
}

void merge_sort(int l , int h , ll (&arr)[S])
{

if(l>=h)
	return ;

ll m=l+(h-l)/2;

merge_sort(l,m,arr);
merge_sort(m+1,h,arr);
merge(l,h,arr);
}

int main()
{
int t;
cin>>t;

while(t--)
{
ans=0;
int n;
cin>>n;

for(int j=0;j<n;j++) 
{
	/**************************************comment 1********
	/int tp;
	/cin>>tp;
	/arr[j]=tp;
	**********************************************************/

	///////////////////////replace following line with comment 1 and you'll get WA
	cin>>arr[j];
}

merge_sort(0,n-1,arr);
cout<<ans<<endl;

}
}

Why do I get WA on replacing?