11110
11010
11000
00000

11000
11000
00100
00011

const dfs = (grid, r, c) => {
  const NR = grid.length;
  const NC = grid[0].length;

  if (r < 0 || c < 0 || r >= NR || c >= NC || grid[r][c] == '0') {
    return;
  }

  grid[r][c] = '0';
  dfs(grid, r - 1, c);
  dfs(grid, r + 1, c);
  dfs(grid, r, c - 1);
  dfs(grid, r, c + 1);
}

/**
 * @param {character[][]} grid
 * @return {number}
 */
const numIslands = grid => {
  if (grid === null || grid.length === 0) {
    return 0;
  }

  const NR = grid.length;
  const NC = grid[0].length;

  let numIslands = 0;
  for (r = 0; r < NR; ++r) {
    for (c = 0; c < NC; ++c) {
      if (grid[r][c] == '1') {
        ++numIslands;
        dfs(grid, r, c);
      }
    }
  }

  return numIslands;
}