Valid Sudoku

MEDIUM✓ CHIP-TIMEDLC #36 — FULL STATEMENT ↗

The drill: Check whether a partly-filled 9×9 sudoku board breaks any rule right now — no repeated digit in a row, column, or 3×3 box. Empty cells are dots and prove nothing.

THE BRIEFING — THE FULL DRILL, IN MY OWN WORDS

A 9×9 board arrives, partially filled with digits and otherwise marked with empty-cell placeholders, and the task is to check whether it currently breaks any sudoku placement rule.

A digit may not repeat within its row, within its column, or within its own 3×3 box — checking those three conditions for every filled cell is the entire job.

Nothing about actually solving the puzzle matters here; an incomplete board with plenty of empty cells left can still be perfectly valid, as long as none of the digits already placed collide.

EX 01

board = ["1........", ".2.......", "..3......", "...4.....", "....5....", ".....6...", "......7..", ".......8.", "........9"]

→ true

CLEAN DIAGONAL

EX 02

board = ["44.......", ".........", ".........", ".........", ".........", ".........", ".........", ".........", "........."]

→ false

ROW REPEAT

EX 03

board = ["7........", "7........", ".........", ".........", ".........", ".........", ".........", ".........", "........."]

→ false

COLUMN REPEAT

the board is always exactly 9 rows by 9 columns
filled cells hold digits 1 through 9; empty cells are a placeholder character
validity only checks current placements, never solvability
the answer is a single true/false verdict

THE HINTS — TAKE ONLY WHAT YOU NEED

HINT 1 THE NUDGE

Nothing needs solving — only catching a repeat. There are exactly three kinds: row, column, box.

HINT 2 THE STRUCTURE

One set per row, per column, per box — 27 small sets. Walk each filled cell once and report the first collision.

HINT 3 ONE STEP FROM THE ANSWER

The box a cell belongs to is (r/3)·3 + c/3 with integer division. One pass, three membership checks per digit.

COACH'S BOARD — THE PATTERN, STEP BY STEP

ROW, COLUMN, BOXPATTERN · ONE PASS, THREE SETS4×4 mini board, 2×2 boxes — a box collision hiding at (1,1)

STEP 1

A 4×4 mini board with 2×2 boxes. Scan cell by cell — row, column, and box sets track what's already placed.

STEP 1 / 6 · ← → WORK TOO

THE SPLITS — TWO PACES, TWO LANGUAGES

grind/valid-sudoku.py — RACE PACE

LANG ▸

PACE ▸

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        rows = [set() for _ in range(9)]
        cols = [set() for _ in range(9)]
        boxes = [set() for _ in range(9)]
        for r in range(9):
            for c in range(9):
                v = board[r][c]
                if v == ".":
                    continue
                b = (r // 3) * 3 + c // 3
                if v in rows[r] or v in cols[c] or v in boxes[b]:
                    return False
                rows[r].add(v)
                cols[c].add(v)
                boxes[b].add(v)
        return True

TIME O(N²)SPACE O(N²)PYTHON · RACE PACE · 17 LN

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        for r in range(9):
            seen = set()
            for c in range(9):
                v = board[r][c]
                if v != ".":
                    if v in seen:
                        return False
                    seen.add(v)
        for c in range(9):
            seen = set()
            for r in range(9):
                v = board[r][c]
                if v != ".":
                    if v in seen:
                        return False
                    seen.add(v)
        for br in range(0, 9, 3):
            for bc in range(0, 9, 3):
                seen = set()
                for r in range(br, br + 3):
                    for c in range(bc, bc + 3):
                        v = board[r][c]
                        if v != ".":
                            if v in seen:
                                return False
                            seen.add(v)
        return True

TIME O(N²)SPACE O(N)PYTHON · TRAINING PACE · 29 LN

class Solution {
    public boolean isValidSudoku(char[][] board) {
        Set<String> seen = new HashSet<>();
        for (int r = 0; r < 9; r++) {
            for (int c = 0; c < 9; c++) {
                char v = board[r][c];
                if (v == '.') {
                    continue;
                }
                int b = (r / 3) * 3 + c / 3;
                if (!seen.add("r" + r + v) || !seen.add("c" + c + v) || !seen.add("b" + b + v)) {
                    return false;
                }
            }
        }
        return true;
    }
}

TIME O(N²)SPACE O(N²)JAVA · RACE PACE · 18 LN

class Solution {
    public boolean isValidSudoku(char[][] board) {
        for (int r = 0; r < 9; r++) {
            Set<Character> seen = new HashSet<>();
            for (int c = 0; c < 9; c++) {
                char v = board[r][c];
                if (v != '.' && !seen.add(v)) {
                    return false;
                }
            }
        }
        for (int c = 0; c < 9; c++) {
            Set<Character> seen = new HashSet<>();
            for (int r = 0; r < 9; r++) {
                char v = board[r][c];
                if (v != '.' && !seen.add(v)) {
                    return false;
                }
            }
        }
        for (int br = 0; br < 9; br += 3) {
            for (int bc = 0; bc < 9; bc += 3) {
                Set<Character> seen = new HashSet<>();
                for (int r = br; r < br + 3; r++) {
                    for (int c = bc; c < bc + 3; c++) {
                        char v = board[r][c];
                        if (v != '.' && !seen.add(v)) {
                            return false;
                        }
                    }
                }
            }
        }
        return true;
    }
}

TIME O(N²)SPACE O(N)JAVA · TRAINING PACE · 36 LN

✓ CHIP-TIMED — ALL 4 SOLUTIONS RAN GREEN AGAINST SELF-AUTHORED CASES IN CI · JDK 21 · CPYTHON 3.12 · NOTHING PUBLISHES RED