P3131 [USACO16JAN] Subsequences Summing to Sevens S

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#include<bits/stdc++.h>
using namespace std;const int N=50010;
int n;
int pos[N];int main()
{ios::sync_with_stdio(0),cin.tie(0);cin>>n;memset(pos,-1,sizeof pos);pos[0]=0;int s=0;int ans=0;for(int i=1;i<=n;i++){int x;cin>>x;s=(s+x)%7;if(pos[s]==-1){pos[s]=i;}else{ans=max(ans,i-pos[s]);}}cout<<ans<<endl;return 0;}