Step through the algorithm visually — use Play or the step buttons (inspired by AlgoMaster / visualgo).
PayPal interview context: Number of Islands is a Medium Graphs problem — BFS for shortest path in unweighted graphs; DFS for connectivity and cycles.
Use the animation above to step through each move before writing code.
Pattern: Graphs
Read from stdin, write to stdout. Classic interview problem #200.
Number of Islands — PayPal interview prep · Graphs
Classic interview problem #200.
Input (stdin)
Line 1: rows\nNext: grid rows (0/1)
Output (stdout)
Island count
Your program must read from stdin and write the answer to stdout (no extra debug text).
4 11110 11010 11000 00000
1
| Test | Status | Details |
|---|
Ready — edit the code above and click Run or Submit.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
class Program
{
static int[] Ria(string line = null)
{
line ??= Console.ReadLine();
if (string.IsNullOrWhiteSpace(line)) return Array.Empty<int>();
return line.Trim().Split(new[] { ' ', ',', '\t' }, StringSplitOptions.RemoveEmptyEntries)
.Select(int.Parse).ToArray();
}
static string[] Rsa()
{
int n = int.Parse(Console.ReadLine());
var arr = new string[n];
for (int i = 0; i < n; i++) arr[i] = Console.ReadLine();
return arr;
}
static void W(params object[] parts) => Console.WriteLine(string.Join(" ", parts));
static void Wb(bool v) => Console.WriteLine(v ? "true" : "false");
static void Wi(int v) => Console.WriteLine(v);
static void Ws(string v) => Console.WriteLine(v);
static void Main()
{
int rows = int.Parse(Console.ReadLine());
var grid = new char[rows][];
for (int r = 0; r < rows; r++) grid[r] = Console.ReadLine().Replace(" ", "").ToCharArray();
int count = 0;
void Dfs(int r, int c) {
if (r < 0 || c < 0 || r >= rows || c >= grid[r].Length || grid[r][c] != '1') return;
grid[r][c] = '0';
Dfs(r + 1, c); Dfs(r - 1, c); Dfs(r, c + 1); Dfs(r, c - 1);
}
for (int r = 0; r < rows; r++)
for (int c = 0; c < grid[r].Length; c++)
if (grid[r][c] == '1') { count++; Dfs(r, c); }
Wi(count);
}
}Try solving on your own first, then reveal the official answer.