$AC$ $Code$

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 500000005;
int pre[MAXN], n, len, maxx = -1, first[7], last[7];
int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> pre[i];
        pre[i] = (pre[i] + pre[i - 1]) % 7;
    }
    for (int i = n; i >= 1; i--)
        first[pre[i]] = i;
    first[0] = 0;
    for (int i = 1; i <= n; i++)
        last[pre[i]] = i;
    for (int i = 0; i <= 6; i++)
        maxx = max(last[i] - first[i], maxx);
    cout << maxx << endl;
    return 0;
}

#include<bits/stdc++.h>
int a[50001],n,ans;
int first[7],last[7];
int main()
{
    std::cin>>n;
    memset(first,-1,sizeof(first));
    memset(last,-1,sizeof(last));
    first[0]=last[0]=0;
    for(int i=1;i<=n;i+=1)
    {
        std::cin>>a[i];
        a[i]=(a[i]+a[i-1])%7;
        if(first[a[i]]==-1)first[a[i]]=i;
        last[a[i]]=i;
    }
    for(int i=0;i<7;i+=1)if(first[i]!=-1)ans=std::max(ans,last[i]-first[i]);
    std::cout<<ans;
}