122 B. Lucky Substring.cpp

#include<bits/stdc++.h>
#define ll              long long
#define ull             unsigned long long
#define pb              push_back
#define fastread()      (ios_base:: sync_with_stdio(false),cin.tie(NULL));
using namespace std;
int main()
{
    fastread();
    //freopen("input.txt","r", stdin);
    string s;
    int n, four = 0, seven = 0, fourty7 = 0;

    cin>>s;
    n = s.length();
    for(int i=0; i<n; i++){
        if(s[i] == '4')four++;
        else if(s[i] == '7')seven++;
        else if((s[i] == '4' && s[i+1] == '7') && (i+1 < n))
            fourty7++;
    }
    if( (four == 0 && seven == 0) && (fourty7 == 0) ){
        cout<<-1<<endl;
    }
    else{
        int ans = 0;
        ans = max(four,max(seven,fourty7));
        //cout<<four<<" "<<seven<<" "<<fourty7<<endl;

        if(ans == four )cout<<4<<endl;
        else if(ans == seven)cout<<7<<endl;
        else if(four == seven)cout<<4<<endl;
        else cout<<47<<endl;
    }

    return 0;
}