SRM403 Div1Easy TheLuckyNumbers
class TheLuckyNumbers { private: int solve(long long n, int a, int b) { if(n > b) return 0; return solve(10*n+4, a, b) + solve(10*n+7, a, b) + (a<=n&&n<=b); } public: int count(int a, int b) { return solve(0, a, b); } };