problems(cpp): add “54. Spiral Matrix”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/spiral-matrix.cpp

@@ -0,0 +1,91 @@
+#include <cassert>
+#include <vector>
+
+class Solution {
+  struct indices {
+    int x;
+    int y;
+
+    bool operator==(const indices& other) const = default;
+    indices& operator+=(const indices& other) {
+      x += other.x;
+      y += other.y;
+      return *this;
+    }
+
+    friend indices operator+(indices left, const indices& right) {
+      return left += right;
+    }
+
+    int operator[](const std::vector<std::vector<int>>& mat) const {
+      return mat[y][x];
+    }
+  };
+
+  struct spiral_indices {
+    spiral_indices(const std::vector<std::vector<int>>& matrix)
+        : x_bounds{-1, static_cast<int>(matrix[0].size())},
+          y_bounds{-1, static_cast<int>(matrix.size())} {}
+
+    bool done() const { return !in_bounds(idx); }
+
+    spiral_indices& operator++() {
+      // update bounds and change the direction if cannot move
+      if (!in_bounds(idx + d)) {
+        // change the direction
+        d = {-d.y, d.x};
+
+        // get the magnitude of the vector
+        auto flat_d = d.x + d.y;
+
+        // decide whether we're moving x or y bounds
+        auto& bounds = d.x ? x_bounds : y_bounds;
+        if (flat_d > 0) {
+          bounds.x++;
+        } else {
+          bounds.y--;
+        }
+      }
+
+      idx += d;
+
+      return *this;
+    }
+
+    int operator[](const std::vector<std::vector<int>>& mat) const {
+      return idx[mat];
+    }
+
+   private:
+    indices idx{0, 0};
+    indices d{1, 0};
+    indices x_bounds, y_bounds;
+
+    bool in_bounds(const indices& idx) const {
+      return (x_bounds.x < idx.x && idx.x < x_bounds.y) &&
+             (y_bounds.x < idx.y && idx.y < y_bounds.y);
+    }
+  };
+
+ public:
+  std::vector<int> spiralOrder(const std::vector<std::vector<int>>& matrix) {
+    std::vector<int> spiral;
+
+    for (spiral_indices idx{matrix}; !idx.done(); ++idx) {
+      spiral.push_back(idx[matrix]);
+    }
+
+    return spiral;
+  }
+};
+
+int main() {
+  Solution s;
+
+  assert((s.spiralOrder({{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}) ==
+          std::vector{1, 2, 3, 6, 9, 8, 7, 4, 5}));
+  assert((s.spiralOrder({{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}) ==
+          std::vector{1, 2, 3, 4, 8, 12, 11, 10, 9, 5, 6, 7}));
+
+  return 0;
+}