31B - Sysadmin Bob - CodeForces Solution

#include <bits/stdc++.h>
#define ll long long
#define all(x) (x).begin(), (x).end()
using namespace std;
bool check(string s){
    int q = 0;
    if (s.size() == 0)
        return true;
    for (int i = 0;i < s.size();i++)
        if (s[i] == '@')
            q++;
    if (q == 1){
        if (s[0] != '@' and s.back() != '@')
            return true;
    }
    return false;
}
int main()
{
    string s;cin>>s;
    for (int i = 1;i < s.size(); i++){
        if (s[i-1] == '@'){
            if (i+1 < s.size() )
                s.insert(i+1, ",");
        }
    }
    for (int i = s.size() -1 ;i >= 0; i--){
        if(s[i]=='@')
            break;
        if (s[i] == ',')
            s.erase(i, 1);
    }
    string temp = "";
    bool ok = true;
    for (int i = 0;i < s.size(); i++){
        if (s[i] == ','){
            if (!check(temp)){
            ok = false;
            break;
            }
            temp = "";
        }else{
            temp += s[i];
        }
    }
    if (!check(temp))
        ok = false;
    if (ok)
        cout<<s;
    else
        cout<<"No solution";
    return 0;
}