add: 220510 [cpp]

cpp/2205/220510.cpp

@@ -0,0 +1,62 @@
+#include <vector>
+#include <functional>
+#include <iostream>
+
+/**
+ * 216. Combination Sum III
+ * Find all valid combinations of k numbers that sum up to n such that the following conditions are true:
+ *
+ * Only numbers 1 through 9 are used.
+ * Each number is used at most once.
+ * Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.
+ */
+
+class Solution {
+public:
+	static std::vector<std::vector<int>> combinationSum3(int k, int n) {
+		int current = 0;
+		std::vector<int> work;
+		std::vector<std::vector<int>> ret;
+		work.reserve(k);
+
+		std::function<void(int, int)> dfs = [&](int rem, int begin) {
+			if (rem <= 1) {
+				if (n - current > work.back() && n - current < 10) {
+					ret.push_back(work);
+					ret.back().push_back(n - current);
+				}
+				return;
+			}
+
+			for (int i = begin; i <= 9; current -= i++ /* On Exit */) {
+				// On Entry
+				current += i;
+
+				// rem is at least 2.
+				if (current + (rem - 1) * (i + 1) + (rem - 1) * (rem - 2) / 2 > n) {
+					current -= i;
+					break;
+				}
+
+				work.push_back(i);
+				dfs(rem - 1, i + 1);
+				work.pop_back();
+			}
+		};
+
+		dfs(k, 1);
+
+		return ret;
+	}
+};
+
+int main() {
+	auto r = Solution::combinationSum3(5, 30);
+	for (const auto& i : r) {
+		for (const auto& j : i) {
+			std::cout << j << ' ';
+		}
+		std::cout << std::endl;
+	}
+	return 0;
+}

cpp/2205/CMakeLists.txt

@@ -3,4 +3,4 @@ PROJECT(2205)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2205 220509.cpp)
+ADD_EXECUTABLE(2205 220510.cpp)