topcoder SRM 593 DIV2 WolfDelaymaster

#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

class WolfDelaymaster{
public:
    bool checkChar(string str, char c, int start ,int num){
        for(int i = start; i < start + num; ++ i ){
            if(str[i] != c) return false;
        }
        return true;
    }

    string check(string str){
        for(int i = 0; i < str.length();){
            int j = i;
            while(str[j] == 'w' && ++ j);
            int num = j - i;
            if(num == 0 ) return "INVALID";
            if(checkChar(str,'0',j,num) && checkChar(str,'l',j+num,num) && checkChar(str,'f',j+2*num,num))
            {
                i = j + 3*num;
            }
            else return "INVALID";
        }
        return "VALID";
    }

};