class TheLuckyNumbers { public: inline static int count(int a, int b) { if (a < 1 || a > 1000000000 || b < 1 || b > 1000000000 || a > b) return 0; return count(b) - count(a - 1); } private: inline static int count(int a) { int result = 0, digits/*=0*/, add = 0; if ((digits = search(a)) != 0) { result = a % digits; a -= result; result += 1; } digits = 1; while (a != 0) { for (int i = 0, j = a % 10; i < j; i++) { result += (i == 4 || i == 7) ? digits : add; } a /= 10; add = add * 8 + digits * 2; digits *= 10; } return result; } private: inline static int search(int a) { int result = 0; for (int digits = 1; a != 0; digits *= 10, a /= 10) { if (a % 10 == 4 || a % 10 == 7) { result = digits; } } return result; } };
