Valid Sudoku

Animated walkthrough

Step through the algorithm visually — use Play or the step buttons (inspired by AlgoMaster / visualgo).

Step 1 / 1

Snap interview context: Valid Sudoku is a Medium Arrays & Hashing problem — Use a hash map to trade space for O(1) lookups — classic at onsite rounds.

Use the animation above to step through each move before writing code.

Pattern: Arrays & Hashing

Read from stdin, write to stdout. Classic interview problem #36.

Problem

Valid Sudoku — Snap interview prep · Arrays & Hashing

Classic interview problem #36.

Input (stdin)

9 lines: board rows (space-separated cells)

Output (stdout)

true or false

Your program must read from stdin and write the answer to stdout (no extra debug text).

Examples

Sample

Input

5 3 . . 7 . . . .
6 . . 1 9 5 . . .
. 9 8 . . . . 6 .
8 . . . 6 . . . 3
4 . . 8 . 3 . . 1
7 . . . 2 . . . 6
. 6 . . . . 2 8 .
. . . 4 1 9 . . 5
. . . . 8 . . 7 9

Output

true

Hints

Input format: 9 lines: board rows (space-separated cells)
DSA Interview 150 — Arrays & Hashing
Problem #36
Frequently asked at Snap
Arrays & Hashing

Your solution

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()
    {
        // Read from stdin (see problem statement for format)
        // 9 lines: board rows (space-separated cells)

}
}

Ready — edit the code above and click Run or Submit.

Solution

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()
    {
var rows = new List<string>();
for (int i = 0; i < 9; i++) rows.Add(Console.ReadLine());
bool ok = true;
var seen = new HashSet<string>();
for (int r = 0; r < 9 && ok; r++) {
    for (int c = 0; c < 9; c++) {
        char ch = rows[r][c * 2];
        if (ch == '.') continue;
        string k1 = $"r{r}{ch}", k2 = $"c{c}{ch}", k3 = $"b{r/3}{c/3}{ch}";
        if (!seen.Add(k1) || !seen.Add(k2) || !seen.Add(k3)) { ok = false; break; }
    }
}
Wb(ok);
    }
}

Try solving on your own first, then reveal the official answer.

Animated walkthrough

Problem

Examples

Your solution

Solution

Discussion