Add day6 in C++

day6.h

@@ -0,0 +1,42 @@
+#include <algorithm>
+#include <fstream>
+#include <iterator>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+namespace {
+  using namespace std;
+
+  auto parse(ifstream input) {
+    unordered_map<string, string> map;
+    string a;
+    while (getline(input, a, ')')) {
+      string b;
+      getline(input, b);
+      map.insert({ b, a });
+    }
+    return map;
+  }
+
+  int part1(ifstream input) {
+    auto map = parse(move(input));
+    int total = 0;
+    for (auto [a, b] : map) {
+      total++;
+      for (; b != "COM"; b = map[b])
+        total++;
+    }
+    return total;
+  }
+
+  int part2(ifstream input) {
+    auto map = parse(move(input));
+    vector<string> you, san;
+    for (auto b = map["YOU"]; b != "COM"; b = map[b])
+      you.push_back(b);
+    for (auto b = map["SAN"]; b != "COM"; b = map[b])
+      san.push_back(b);
+    return you.size() + san.size() - 2 * distance(rbegin(you), mismatch(rbegin(you), rend(you), rbegin(san)).first);
+  }
+}