Add 2021 day 9 puzzles.

2021/puzzle-09-01.cc

@@ -0,0 +1,35 @@
+//
+// Created by mgretton on 09/12/2021.
+//
+
+#include <iostream>
+#include <string>
+#include <vector>
+
+using Depths = std::vector<std::string>;
+
+auto main() -> int
+{
+  Depths depths;
+  std::string line;
+  while (std::getline(std::cin, line)) {
+    depths.push_back(line);
+  }
+
+  unsigned risk{0};
+  for (std::size_t y{0}; y < depths.size(); ++y) {
+    for (std::size_t x{0}; x < depths[y].size(); ++x) {
+      bool min = true;
+      min &= (x == 0) || (depths[y][x - 1] > depths[y][x]);
+      min &= (y == 0) || (depths[y - 1][x] > depths[y][x]);
+      min &= (x == depths[y].size() - 1) || (depths[y][x + 1] > depths[y][x]);
+      min &= (y == depths.size() - 1) || (depths[y + 1][x] > depths[y][x]);
+      if (min) {
+        risk += depths[y][x] - '0' + 1;
+      }
+    }
+  }
+
+  std::cout << "Risk factor " << risk << '\n';
+  return 0;
+}

2021/puzzle-09-02.cc

@@ -0,0 +1,74 @@
+//
+// Created by mgretton on 09/12/2021.
+//
+
+#include <iostream>
+#include <set>
+#include <string>
+#include <vector>
+
+using Depths = std::vector<std::string>;
+using Coord = std::pair<std::size_t, std::size_t>;
+using Coords = std::set<Coord>;
+
+auto find_basin_size(std::size_t x1, std::size_t y1, Depths const& depths) -> std::size_t
+{
+  Coords visited;
+  Coords to_visit;
+  to_visit.insert({x1, y1});
+  while (!to_visit.empty()) {
+    Coords next_to_visit;
+    for (auto const& coord : to_visit) {
+      auto x{coord.first};
+      auto y{coord.second};
+      if (x > 0 && depths[y][x - 1] != '9' && !visited.contains({x - 1, y})) {
+        next_to_visit.insert({x - 1, y});
+        visited.insert({x - 1, y});
+      }
+      if (y > 0 && depths[y - 1][x] != '9' && !visited.contains({x, y - 1})) {
+        next_to_visit.insert({x, y - 1});
+        visited.insert({x, y - 1});
+      }
+      if (x < depths[y].size() - 1 && depths[y][x + 1] != '9' && !visited.contains({x + 1, y})) {
+        next_to_visit.insert({x + 1, y});
+        visited.insert({x + 1, y});
+      }
+      if (y < depths.size() - 1 && depths[y + 1][x] != '9' && !visited.contains({x, y + 1})) {
+        next_to_visit.insert({x, y + 1});
+        visited.insert({x, y + 1});
+      }
+    }
+    std::swap(next_to_visit, to_visit);
+  }
+
+  return visited.size();
+}
+
+auto main() -> int
+{
+  Depths depths;
+  std::string line;
+  while (std::getline(std::cin, line)) {
+    depths.push_back(line);
+  }
+
+  std::vector<std::size_t> basin_sizes;
+  for (std::size_t y{0}; y < depths.size(); ++y) {
+    for (std::size_t x{0}; x < depths[y].size(); ++x) {
+      bool min = true;
+      min &= (x == 0) || (depths[y][x - 1] > depths[y][x]);
+      min &= (y == 0) || (depths[y - 1][x] > depths[y][x]);
+      min &= (x == depths[y].size() - 1) || (depths[y][x + 1] > depths[y][x]);
+      min &= (y == depths.size() - 1) || (depths[y + 1][x] > depths[y][x]);
+      if (min) {
+        basin_sizes.push_back(find_basin_size(x, y, depths));
+      }
+    }
+  }
+
+  std::sort(basin_sizes.begin(), basin_sizes.end());
+  std::reverse(basin_sizes.begin(), basin_sizes.end());
+  std::cout << "Result " << basin_sizes[0] << " * " << basin_sizes[1] << " * " << basin_sizes[2]
+            << " = " << basin_sizes[0] * basin_sizes[1] * basin_sizes[2] << '\n';
+  return 0;
+}