Add 2016 day 19 puzzles

2016/puzzle-19-01.cc

@@ -0,0 +1,36 @@
+//
+// Created by Matthew Gretton-Dann on 07/12/2021.
+//
+
+#include <iostream>
+#include <list>
+#include <string>
+
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read input.\n";
+    return 1;
+  }
+  auto n{std::stoul(line)};
+  std::list<unsigned long> elves;
+  for (unsigned long i{1}; i <= n; ++i) {
+    elves.push_back(i);
+  }
+  auto it{elves.begin()};
+  while (elves.size() > 1) {
+    std::cout << *it << " takes ";
+    ++it;
+    if (it == elves.end()) {
+      it = elves.begin();
+    }
+    std::cout << *it << '\n';
+    it = elves.erase(it);
+    if (it == elves.end()) {
+      it = elves.begin();
+    }
+  }
+
+  std::cout << "Last elf: " << elves.front() << '\n';
+}

2016/puzzle-19-02.cc

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+//
+// Created by Matthew Gretton-Dann on 07/12/2021.
+//
+
+#include <iostream>
+#include <list>
+#include <string>
+
+/* Consider the case of 8 elves:
+ *  Start: 1 2 3 4 5 6 7 8
+ *  1 -> 5: 1 2 3 4 6 7 8
+ *  2 -> 6: 1 2 3 4 7 8
+ *  3 -> 8: 1 2 3 4 7
+ *  4 -> 1: 2 3 4 7
+ *  7 -> 3: 2 4 7
+ *  2 -> 4: 2 7
+ *  7 -> 2: 7
+ *
+ *  Looking at this we can see that we start by removing the element half-way round the list.  The
+ *  next element removed is always either the one after the one just removed, or the one just beyond
+ *  that.
+ *
+ *  Which one is removed depends on whether there are an even or odd number of elements left.
+ *
+ *  We make use of this to realize we don't have to track the current elf who is getting parcels
+ *  only the elf that is having parcels stolen from them.
+ */
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read input.\n";
+    return 1;
+  }
+  auto n{std::stoul(line)};
+  std::list<unsigned long> elves;
+  for (unsigned long i{1}; i <= n; ++i) {
+    elves.push_back(i);
+  }
+  /* it points to the first elf to remove.  */
+  auto it{elves.begin()};
+  for (unsigned long i{0}; i < n / 2; ++i) {
+    ++it;
+  }
+  while (n > 1) {
+    it = elves.erase(it);
+    if (it == elves.end()) {
+      it = elves.begin();
+    }
+    if ((--n) % 2 == 0) {
+      ++it;
+    }
+    if (it == elves.end()) {
+      it = elves.begin();
+    }
+  }
+
+  std::cout << "Last elf: " << elves.front() << '\n';
+}