Flood fill / islands (Graph BFS / DFS Sub-pattern)

// Flood fill: every unvisited land cell starts one DFS; everything it reaches is one island.
int dr[4] = {1, -1, 0, 0}, dc[4] = {0, 0, 1, -1};

int sink(vector<vector<char>>& grid, int r, int c) {
    int R = grid.size(), C = grid[0].size();
    if (r < 0 || c < 0 || r >= R || c >= C || grid[r][c] != '1') return 0;
    grid[r][c] = '0';                             // mark BEFORE exploring, so no cell is counted twice
    int size = 1;
    for (int d = 0; d < 4; d++) size += sink(grid, r + dr[d], c + dc[d]);
    return size;                                  // island size (Max Area); ignore it to just count
}

int countIslands(vector<vector<char>>& grid) {
    int islands = 0;
    for (int r = 0; r < (int)grid.size(); r++)
        for (int c = 0; c < (int)grid[0].size(); c++)
            if (grid[r][c] == '1') { islands++; sink(grid, r, c); }
    return islands;
}