Number of Islands

You are given a rectangular grid of characters, where '1' means land and '0' means water. An island is a group of '1's connected horizontally or vertically (not diagonally). Count how many islands there are and return that number. Input: `grid` (a list of lists of '1' and '0' characters). Output: the number of islands. Example: grid = [["1", "1", "0"], ["0", "1", "0"], ["0", "0", "0"]] -> 1.

def num_islands(grid: list[list[str]]) -> int:
    """Count the islands of '1's connected horizontally or vertically."""
    pass