void merge(int *a,int p,int q,int r){
    int n1 = q-p+1;
    int n2 = r-q;
    int *m = new int[n1];
    int *n = new int[n2];
    for(int i=0;i<n1;i++) m[i] = a[p+i];
    for(int j=0;j<n2;j++) n[j] = a[q+j+1];
    int i=0,j=0,k=p;
    while(i<n1&&j<n2){
        if(m[i]<=n[j]) a[k++] = m[i++];
        else a[k++] = n[j++];
    }
    while(i<n1) a[k++] = m[i++];
    while(j<n2) a[k++] = n[j++];
    delete[] m,n;
}
void mergeSort(int *a,int p,int r){
    if(p<r){
        int q = (p+r)/2;
        mergeSort(a,p,q);
        mergeSort(a,q+1,r);
        merge(a,p,q,r);
    }
}