Valid Sudoku

Problem

Determine if a 9 x 9 Sudoku board is valid according to the following rules: each row, column, and 3x3 sub-box must contain the digits 1-9 without repetition.

Examples

The brute-force approach

For each row, column, and 3×3 box, collect non-empty values into an array and check for duplicates by sorting or using a set.

for each row in board:
    check no duplicates among non-'.' values
for each col index in 0..8:
    check no duplicates in that column
for each box (row/3, col/3) in 0..2 × 0..2:
    check no duplicates in that box

This is already O(81) = O(1) since the board is fixed size. The question is about clean implementation.

Spotting the pattern

This is a Set problem. The key question to ask yourself:

For a cell at row r and column c, which 3×3 box does it belong to? How do you compute a unique index for that box?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.