Problem description
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.
Example 1:
Input: 11110 11010 11000 00000
Output: 1 Example 2:
Input: 11000 11000 00100 00011
Output: 3
Solution
We can use DFS to change all connected 1’s to X, and count groups of connected 1’s.
Below is my python implementation
class Solution:
def numIslands(self, grid: List[List[str]]) -> int:
count = 0
m = len(grid)
if m == 0:
return 0
n = len(grid[0])
def neighbors(x:int, y:int)->List[List[int]]:
ans = []
if x + 1 < m and grid[x + 1][y] == '1':
ans.append([x + 1, y])
if x - 1 >= 0 and grid[x - 1][y] == '1':
ans.append([x - 1, y])
if y + 1 < n and grid[x][y + 1] == '1':
ans.append([x, y + 1])
if y - 1 >= 0 and grid[x][y - 1] == '1':
ans.append([x, y - 1])
return ans
def dfs(i:int, j:int)->None: #mark 1's with X. When finish on island, add count by 1
grid[i][j] = 'X'
for neighbor in neighbors(i, j):
x, y = neighbor
dfs(x, y)
return
for i in range(m):
for j in range(n):
if grid[i][j] == '1':
dfs(i, j)
count += 1
return count