add: 230316-CN

cpp/2303/230316-CN.cpp

@@ -0,0 +1,44 @@
+#include <vector>
+#include <unordered_map>
+#include <iostream>
+
+/**
+ * 2488. Count Subarrays With Median K
+ *
+ * You are given an array nums of size n consisting of distinct integers from 1 to n and a positive integer k.
+ *
+ * Return the number of non-empty subarrays in nums that have a median equal to k.
+ *
+ * Note:
+ *
+ * The median of an array is the middle element after sorting the array in ascending order. If the array is of even length, the median is the left middle element.
+ * For example, the median of [2,3,1,4] is 2, and the median of [8,4,3,5,1] is 4.
+ * A subarray is a contiguous part of an array.
+ */
+
+class Solution {
+private:
+	static inline constexpr int sgn(int x) { return (x > 0) - (x < 0); }
+
+public:
+	static int countSubarrays(const std::vector<int>& nums, int k) {
+		const int n = nums.size(), pos = std::find(nums.begin(), nums.end(), k) - nums.begin();
+		std::vector<int> prefix(n);
+		prefix[0] = sgn(nums[0] - k);
+		for (int i = 1; i < n; ++i)
+			prefix[i] = prefix[i - 1] + sgn(nums[i] - k);
+		std::unordered_map<int, int> m;
+		for (int i = 0; i < pos; ++i)
+			++m[prefix[i]];
+		++m[0];
+		int ans = 0;
+		for (int i = pos; i < n; ++i)
+			ans += m[prefix[i]] + m[prefix[i] - 1];
+		return ans;
+	}
+};
+
+int main() {
+	std::cout << Solution::countSubarrays({3,2,1,4,5}, 4);
+	return 0;
+}

cpp/2303/CMakeLists.txt

@@ -3,4 +3,4 @@ PROJECT(2303)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2303 230315-CN.cpp)
+ADD_EXECUTABLE(2303 230316-CN.cpp)