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;
+}