归并排序20个点全TLE

各位大佬，我的代码是归并排序 O(n log n) ，为什么 TLE 了

#pragma G++ optimize(3)
#include <bits/stdc++.h>
using namespace std;

long long n,a[1000002],b[1000002],ans=0;

void msort(long long l,long long r)
{
    if (l==r) return;
    int mid=(l+r)/2;
    msort(1,mid);
    msort(mid+1,r);
    long long i=l,j=mid+1,p=l;
    while (i<=mid&&j<=r)
    {
        if (a[i]<=a[j]) b[p++]=a[i++];
        else b[p++]=a[j++],ans+=mid-i+1;
    }
    while (j<=r) b[p++]=a[j++];
    while (i<=mid) b[p++]=a[i++];
    for (long long k=l;k<=r;k++) a[k]=b[k];
}
int main()
{
    scanf("%lld",&n);
    for(long long i=1;i<=n;i++) scanf("%lld",&a[i]);
    msort(1,n);
    printf("%lld",ans);
    //for (int i=1;i<=n;i++) printf("%d ",a[i]);
    printf("\n");
    return 0;
}