Number of Islands - ELI5 Park

👶 The 5-Year-Old Summary

You have a map of the ocean (0s) and land (1s). Count how many islands (connected land pieces)! Connected means touching up, down, left, or right - no diagonals!

🎮 Interactive Visualizer

🧠 The Brain Hack

DFS flood fill! When you find land, "sink" it (turn to water) and recursively sink all connected land. Each time you start a new DFS = new island!

💻 The Clean Solution

def num_islands(grid):
    if not grid: return 0
    
    def dfs(r, c):
        # Out of bounds or water = stop
        if (r < 0 or c < 0 or 
            r >= len(grid) or c >= len(grid[0]) or
            grid[r][c] == '0'):
            return
        
        # "Sink" this land
        grid[r][c] = '0'
        
        # Visit all 4 directions
        dfs(r+1, c)
        dfs(r-1, c)
        dfs(r, c+1)
        dfs(r, c-1)
    
    count = 0
    for r in range(len(grid)):
        for c in range(len(grid[0])):
            if grid[r][c] == '1':
                count += 1
                dfs(r, c)  # Sink the whole island
    
    return count

# 1 1 1 0 0
# 1 1 0 0 1
# 0 0 0 1 1
# 0 0 0 0 0
# Answer: 3 islands

Time: O(m × n)

Space: O(m × n) worst case