Codeforces Round #496 (Div. 3), problem: (D) Polycarp and Div 3

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define pii pair<int,int>
#define pli pair<long long int,int>
#define pbi pair<bool,int>
#define pob pop_back
#define all(a) a.begin(),a.end()
#define fio ios_base::sync_with_stdio(false); cin.tie(0);cout.tie(0);
#define i64 long long int
#define mem(x,y) memset(x,y,sizeof(x))
#define fill(arr,b) fill(arr, arr+sizeof(arr)/sizeof(arr[0]), b)
#define maxarr(n,arr,m) m =-1e9;for(int i=0;i<n;i++)if(arr[i]>m) m=arr[i];
#define read freopen("input.txt","r",stdin);

const double pi = acos(-1.0);
const int maxn = (i64)2e9+5;

int sum, ans, rem[3];
void reset()
{
    rem[0] = 1;
    rem[2] = rem[1] = 0;
    sum = 0;
}
int main()
{
    reset();
    string s;
    cin >> s;
    int n = s.size();
    for(int i = 0 ; i<n ; i++)
    {
        sum = (sum + (s[i]-48))%3;
        rem[sum]++;
        if(rem[sum]>1) {reset();ans++;}
    }
    cout << ans << endl;
return 0;
}