Merge sorting

code:--

#include<iostream>
#include<algorithm>
using namespace std;
void merge(int arr[],int p,int q,int r)
{
int n1=r-p+1;
int n2=q-r;
int a[n1],b[n2];
int i;
i=p;
while(i<=r)
{
a[i-p]=arr[i];
i++;
}
while(i<=q)
{
b[i-r-1]=arr[i];
i++;
}
i=p;
int x=0,y=0;
while(x<n1 && y<n2)
    if(a[x]<=b[y])
arr[i++]=a[x++];
else
arr[i++]=b[y++];
while(x<n1)
arr[i++]=a[x++];
while(y<n2)
arr[i++]=b[y++];
}
void mergesort(int arr[],int p,int q)
{
if(p==q)
return;
int r=(p+q)/2;
mergesort(arr,p,r);
mergesort(arr,r+1,q);
merge(arr,p,q,r);
}
int main()
{
int n;
cin>>n;
int a[n];
for(int i=0;i<n;i++)
cin>>a[i];
std::cout<<"Sorted elements are:\n";
mergesort(a,0,n-1);
for(int i=0;i<n;i++)
cout<<a[i]<<" ";
return 0;
}