Number of Islands

The idea is to count each land as a new island but when a land is counted a function will set this land and all the linked lands to 'already seen'.

def numIslands(grid: List[List[str]]) -> int:
    # dfs set all of the land linked to the current land (if it is land) to 'already seen' (#)
    # dfs does if by recursion. it stops if current grid is water or border
    def dfs(grid, i, j):
        if i<0 or j<0 or i>=len(grid) or j>=len(grid[0]) or grid[i][j] != '1':
            return
        grid[i][j] = '#'
        dfs(grid, i+1, j)
        dfs(grid, i-1, j)
        dfs(grid, i, j+1)
        dfs(grid, i, j-1)

    # if grid is emtpy then there is no island
    if not grid:
        return 0

    island_count = 0
    for i in range(len(grid)):
        for j in range(len(grid[0])):
            # if a grid value is 1 (land), add 1 to the island counter and set all of the linked lands to already counter (#)
            if grid[i][j] == '1':
                dfs(grid, i, j) # set all related land (and this one) to 'already seen'
                island_count += 1 # update the island counter
    return island_count