Added older advents

advent_of_code_2018/day1/main2.cc

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+#include <iostream>
+#include <set>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+int read()
+{
+    string line;
+    getline(cin, line);
+
+    bool plus = line[0] == '+';
+    int val = stoi(line.substr(1));
+
+    return plus ? val : -val;
+}
+
+int main()
+{
+    vector<int> sequence;
+    while (!cin.eof())
+        sequence.push_back(read());
+
+    // cout << "Buffered " << sequence.size() << " frequencies\n";
+    
+    int total = 0;
+    set<int> used;
+    used.insert(0);
+
+    while (true)
+    {
+        for (int i : sequence)
+        {
+            total += i;
+
+            if (used.find(total) != used.end())
+            {
+                cout << "Found first repeating frequency: " << total << "\n";
+                return 0;
+            }
+
+            used.insert(total);
+        }
+    }
+}