add: 211230

2112/211230.cpp

@@ -0,0 +1,35 @@
+#include <iostream>
+#include <unordered_set>
+
+class Solution {
+public:
+	static int smallestRepunitDivByK(int k) {
+		if (k == 1) return 1;
+		/*
+		 * Bad code.
+		 * Despite std::unordered_set is O(1),
+		 * it increases space complexity
+		 * and the constant increases.
+		int pos = 2, rem = 11 % k;
+		std::unordered_set<int> s;
+		s.insert(rem);
+		while (true) {
+			if (!rem)
+				return pos;
+			rem = (10 * rem + 1) % k;
+			++pos;
+			if (s.count(rem))
+				return -1;
+			s.insert(rem);
+		}
+		 */
+		for (int pos = 1, rem = 1; pos < k; ++pos)
+			if (!(rem = (10 * rem + 1) % k))
+				return ++pos;
+		return -1;
+	}
+};
+
+int main() {
+	std::printf("%d\n", Solution::smallestRepunitDivByK(7));
+}

2112/CMakeLists.txt

@@ -3,4 +3,4 @@ PROJECT(2112)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2112 211229.cpp)
+ADD_EXECUTABLE(2112 211230.cpp)