Added older advents

advent_of_code_2018/day2/main1.cc

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+#include <algorithm>
+#include <iostream>
+#include <string>
+
+using namespace std;
+
+int main()
+{
+    size_t dubbles = 0;
+    size_t tripples = 0;
+
+    string buffer;
+
+    while(getline(cin, buffer))
+    {
+        bool twice = false;
+        bool thrice = false;
+        size_t letters[26] = { };
+
+        for (char c : buffer)
+        {
+            ++letters[c - 'a'];
+        }
+
+        for (size_t idx = 0; idx < 26; ++idx)
+        {
+            if (letters[idx] == 2)
+                twice = true;
+            if (letters[idx] == 3)
+                thrice = true;
+        }
+
+        if (twice)
+            ++dubbles;
+        if (thrice)
+            ++tripples;
+    }
+
+    cout << "checsum: " << dubbles * tripples << "\n";
+}