HackerEarth LongLong problem solution

#include<bits/stdc++.h>
 
using namespace std;
 
string st;
long long pw[1<<20];
unordered_set <long long> S;
long long s[1<<20];
 
int check(int span)
{
    S.clear();
    
    for (int i=0;i+span<=st.size();i++)
    {
        long long Q=s[i+span]-s[i];
        Q*=pw[1000000-i];
        if (S.find(Q)!=S.end())
            return true;
        S.insert(Q);
    }
    
    return false;
}
 
int main(){
ios_base::sync_with_stdio(0);
 
cin>>st;

pw[0]=1;
for (int i=1;i<=1000000;i++)
    pw[i]=pw[i-1]*173;
 
for (int i=1;i<=st.size();i++)
    s[i]=s[i-1]+st[i-1]*pw[i-1];
 
int l,r;
l=0;
r=st.size();
while (l<r)
{
    int mid=l+r+1;
    mid/=2;
    if (check(mid))
        l=mid;
    else
        r=mid-1;
}
 
cout<<l<<endl;

return 0;}