ACM merge sort

#include <iostream>
using namespace std;

int n;

void mergeSort(int a[], int l, int r){
    if (l==r)
        return;
    int mid=(l+r)/2;
    mergeSort(a, l, mid);
    mergeSort(a, mid+1, r);
    int i, x=l, y=mid+1, t[r-l+1];
    for (i=0; i<=r-l; i++){
        if (y>r || (x<=mid && a[x]<a[y])){
            t[i]=a[x++];
        } else {
            t[i]=a[y++];
        }
    }
    for (i=0; i<=r-l; i++){
        a[l+i]=t[i];
    }
}


int main() {
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    cin>>n;
    int a[n], i;
    for (i=0; i<n; i++){
        cin>>a[i];
    }
    mergeSort(a, 0, n-1);
    for (i=0; i<n; i++){
        cout<<a[i]<<" ";
    }
    return 0;
}