Number of Islands
Created: October 9, 2018 by [lek-tin]
Last updated: April 17, 2020
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
DFS
Solution (DFS in Java)
DFS
class Solution {
public int numIslands(char[][] grid) {
if (grid.length == 0 || grid[0].length == 0) {
return 0;
}
int res = 0;
for (int i = 0; i < grid.length; i++) {
for (int j = 0; j < grid[0].length; j++) {
if (grid[i][j] == '1') {
dfs(grid, i, j);
res++;
}
}
}
return res;
}
private void dfs(char[][] grid, int i, int j) {
int[][] dirs = {
{0,1},
{0,-1},
{1,0},
{-1, 0}}
;
if (i < 0 || i >= grid.length || j < 0 || j >= grid[0].length || grid[i][j] != '1') {
return;
}
grid[i][j] = 'x';
for (int[] dir: dirs) {
int dx = dir[0];
int dy = dir[1];
dfs(grid, i+dx, j+dy);
}
}
}
Solution (DFS in Python)
DFS
class Solution:
def numIslands(self, grid):
"""
:type grid: List[List[str]]
:rtype: int
"""
# Edge cases
if not grid or len(grid) == 0 or len(grid[0]) == 0:
return 0
rows = len(grid)
cols = len(grid[0])
count = 0
for i in range(rows):
for j in range(cols):
if grid[i][j] == "1":
count += 1
self.markNeighbours(grid, rows, cols, i, j)
return count
def markNeighbours(self, grid, rows, cols, x, y):
# When exceeds beyond boundaries or current point was visited, return
if x < 0 or x >= rows or y < 0 or y >= cols or grid[x][y] == "0" or grid[x][y] == "2":
return
# Mark this point as visited
grid[x][y] = "2"
directions = [ [-1, 0], [1, 0], [0, -1], [0, 1] ]
for dir in directions:
adjacentX = x + dir[0]
adjacentY = y + dir[1]
self.markNeighbours(grid, rows, cols, adjacentX, adjacentY)
BFS
from collections import deque
class Solution:
def numIslands(self, grid: List[List[str]]) -> int:
res = 0
if len(grid) == 0 or len(grid[0]) == 0:
return res
for i in range(len(grid)):
for j in range(len(grid[0])):
if grid[i][j] == "1":
self.search(i, j, grid)
res += 1
return res
def inBound(self, x, y, grid):
return x >= 0 and x < len(grid) and y >= 0 and y < len(grid[0])
def search(self, i, j, grid):
directions = [ [0, 1], [0, -1], [1, 0], [-1, 0] ]
q = deque()
q.append( (i, j) )
grid[i][j] = "-1"
while q:
x, y = q.popleft()
for dir in directions:
new_x, new_y = x+dir[0], y+dir[1]
if not self.inBound(new_x, new_y, grid):
continue
if grid[new_x][new_y] == "1":
grid[new_x][new_y] = "-1"
q.append( (new_x, new_y) )