Valid Sudoku: the idea to remember.

The core insight

Track seen digits separately for each row, column and 3 by 3 box; skip empty cells.

Why it works

A board is valid exactly when no filled digit repeats in any of its three containing units.

The mistake to avoid

A valid partial board need not have a complete solution. Ignore dots rather than treating them as digits.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

// Java reference adapted from neetcode-gh/leetcode (MIT).
// Track seen digits separately for each row, column and 3 by 3 box; skip empty cells.
public boolean isValidSudoku(char[][] board) {



    Set<Character> rowSet = null;
    Set<Character> colSet = null;


    for (int i = 0; i < 9; i++) {

        rowSet = new HashSet<>();
        colSet = new HashSet<>();
        for (int j = 0; j < 9; j++) {
            char r = board[i][j];
            char c = board[j][i];
            if (r != '.'){
                if (rowSet.contains(r)){
                    return false;
                } else {
                    rowSet.add(r);
                }
            }
            if (c != '.'){
                if (colSet.contains(c)){
                    return false;
                } else {
                    colSet.add(c);
                }
            }
        }
    }



    for (int i = 0; i < 9; i = i + 3) {
        for (int j = 0; j < 9; j = j + 3) {

            if (!checkBlock(i, j, board)) {
                return false;
            }
        }
    }

    return true;
}

public boolean checkBlock(int idxI, int idxJ, char[][] board) {
    Set<Character> blockSet = new HashSet<>();


    int rows = idxI + 3;
    int cols = idxJ + 3;

    for (int i = idxI; i < rows; i++) {

        for (int j = idxJ; j < cols; j++) {
            if (board[i][j] == '.') {
                continue;
            }

            if (blockSet.contains(board[i][j])) {
                return false;
            }

            blockSet.add(board[i][j]);
        }
    }

    return true;
}

Java reference adapted from neetcode-gh/leetcode · MIT license

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