1 files changed, 41 insertions, 0 deletions

diff --git a/2025/src/day04.cpp b/2025/src/day04.cpp
index ef3b7f5..75c86d9 100644
--- a/2025/src/day04.cpp
+++ b/2025/src/day04.cpp
@@ -94,6 +94,46 @@ part2(set<Pos> grid)
         cout << "Part 2: " << old_size - grid.size() << '\n';
 }
 
+void
+part2_bfs(set<Pos> grid)
+{
+        const auto old_size = grid.size();
+
+        map<Pos, size_t> counts;
+        for ( const auto& pos: grid ) {
+                counts[pos] = get_nneighbours(grid, pos);
+        }
+
+        queue<Pos> queue;
+        for ( const auto& [pos, count]: counts ) {
+                if ( count < 4 ) {
+                        queue.emplace(pos);
+                }
+        }
+
+        while ( !queue.empty() ) {
+                const auto pos = queue.front();
+                queue.pop();
+
+                if ( !grid.contains(pos) ) {
+                        continue;
+                }
+
+                grid.erase(pos);
+
+                for ( const auto& neighbour: get_neighbours(pos) ) {
+                        if ( !grid.contains(neighbour) ) {
+                                continue;
+                        }
+                        if ( --counts[neighbour] == 3 ) {
+                                queue.emplace(neighbour);
+                        }
+                }
+        }
+
+        cout << "Part 2: " << old_size - grid.size() << '\n';
+}
+
 } // namespace
 
 int
@@ -102,4 +142,5 @@ main()
         auto grid = read_file("data/day04.txt");
         part1(grid);
         part2(grid);
+        part2_bfs(grid);
 }