Added older advents

advent_of_code_2020/day1/main2.cc

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+#include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+int main()
+{
+    vector<int> numbers;
+    ifstream in("input.txt");
+
+    while (true)
+    {
+        int tmp;
+        in >> tmp;
+
+        if (!in)
+            break;
+
+        numbers.push_back(tmp);
+        cout << "read " << tmp << "\n";
+    }
+
+    cout << "Read " << numbers.size() << " numbers.\n";
+
+    for (size_t outer = 0; outer < numbers.size(); ++outer)
+    {
+        for (size_t inner = outer + 1; inner < numbers.size(); ++inner)
+        {
+            for (size_t innest = inner + 1; innest < numbers.size(); ++innest)
+            {
+                if (numbers[outer] + numbers[inner] + numbers[innest] == 2020)
+                    cout << "Answer: " << numbers[outer] * numbers[inner] * numbers[innest] << "\n";
+
+            }
+        }
+    }
+
+
+
+}