number_islands.py

'''
Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. 
An island is surrounded by water and is formed by connecting adjacent lands horizontally 
or vertically. You may assume all four edges of the grid are all surrounded by water.
Input:
11000
11000
00100
00011

Output: 3
'''
def numIslands(grid):
        if not grid:
            return 0
        count = 0
        for i in range(0,len(grid)):
            for j in range(0, len(grid[0])):
                if grid[i][j] == '1':
                    dfs(grid,i,j)
                    count+=1
        return count

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] = 'v'
    dfs(grid,i-1,j)
    dfs(grid,i,j-1)
    dfs(grid,i+1,j)
    dfs(grid,i,j+1)

print(numIslands([["1","1","1","1","0"],["1","1","0","1","0"],["1","1","0","0","0"],["0","0","0","0","0"]]))