Added part one of day four, plus a few 2020's from my laptop.

2020/1/main.cpp

@@ -0,0 +1,43 @@
+#include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
+
+int find2(std::vector<int> &numbers)
+{
+	for(auto i = 0; i < numbers.size(); ++i)
+		for(auto j = i + 1; j < numbers.size(); ++j)
+			if(numbers[i] + numbers[j] == 2020)
+				return numbers[i] * numbers[j];
+
+	return 0;
+}
+int find3(std::vector<int> &numbers)
+{
+	for(auto i = 0; i < numbers.size(); ++i)
+		for(auto j = i + 1; j < numbers.size(); ++j)
+			for(auto k = i + 1; k < numbers.size(); ++k)
+				if(numbers[i] + numbers[j] + numbers[k] == 2020)
+					return numbers[i] * numbers[j] * numbers[k];
+
+	return 0;
+}
+
+int main()
+{
+	std::ifstream ifs("data");
+	if(!ifs.is_open())
+	{
+		std::cerr << "Couldn't open file." << std::endl;
+		return -1;
+	}
+
+	std::vector<int> numbers;
+	for(std::string line; std::getline(ifs, line); )
+		numbers.push_back(std::atoi(line.c_str()));
+
+	std::cout << find2(numbers) << std::endl;
+	std::cout << find3(numbers) << std::endl;
+
+	return 0;
+}