18-miniron-v

miniron-v/README.md

@@ -20,4 +20,5 @@
 | 15차시 | 2024.02.15 |  DP, 큰 수 연산  | [타일링](https://www.acmicpc.net/problem/1793)  | [#56](https://github.com/AlgoLeadMe/AlgoLeadMe-5/pull/56) |
 | 16차시 | 2024.02.18 |  수학  | [합](https://www.acmicpc.net/problem/1081)  | [#61](https://github.com/AlgoLeadMe/AlgoLeadMe-5/pull/61) |
 | 17차시 | 2024.02.21 |  DP  | [제곱수의 합](https://www.acmicpc.net/problem/1699)  | [#64](https://github.com/AlgoLeadMe/AlgoLeadMe-5/pull/64) |
+| 18차시 | 2024.02.24 |  이분 탐색  | [K번째 수](https://www.acmicpc.net/problem/1300)  | [#66](https://github.com/AlgoLeadMe/AlgoLeadMe-5/pull/66) |
 ---

miniron-v/이분 탐색/1300-K번째 수.cpp

@@ -0,0 +1,35 @@
+#include <iostream>
+
+// [n][n] 배열에서 upper 이하 수의 개수를 센다.
+long countNum(long n, long upper) {
+	long count = 0;
+
+	for (long i = 1; i <= n && i <= upper; ++i) {
+		count += std::min(upper / i, n);
+	}
+
+	return count;
+}
+
+int main() {
+	long n, k;
+	std::cin >> n >> k;
+
+	long low = 0, high = k;
+	long mid = 0;
+
+	while (low + 1 < high) {
+		mid = (low + high) / 2;
+		long count = countNum(n, mid);
+
+		if (count < k) {
+			low = mid;
+		}
+
+		else if (count >= k) {
+			high = mid;
+		}
+	}
+
+	std::cout << high;
+}