CodeEasy v0.1

Number of Islands

Medium Subject: Graphs
Time Complexity
O(M * N)
Space Complexity
O(M * N)

Problem Description

Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands.

An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically.

Optimal Solution

Python 
def numIslands(grid):
    if not grid:
        return 0

    m, n = len(grid), len(grid[0])
    islands = 0

    def dfs(r, c):
        if r < 0 or c < 0 or r >= m or c >= n or grid[r][c] == '0':
            return
        grid[r][c] = '0'  # Mark as visited
        dfs(r + 1, c)
        dfs(r - 1, c)
        dfs(r, c + 1)
        dfs(r, c - 1)

    for r in range(m):
        for c in range(n):
            if grid[r][c] == '1':
                islands += 1
                dfs(r, c)

    return islands