validsudoku.java

import java.util.HashSet;

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        // HashSet to track the numbers seen in rows, columns, and sub-grids
        for (int i = 0; i < 9; i++) {
            HashSet<Character> rowSet = new HashSet<>();
            HashSet<Character> colSet = new HashSet<>();
            HashSet<Character> subGridSet = new HashSet<>();

            for (int j = 0; j < 9; j++) {
                // Check the row
                if (board[i][j] != '.' && !rowSet.add(board[i][j])) {
                    return false; // Duplicate in row
                }
                // Check the column
                if (board[j][i] != '.' && !colSet.add(board[j][i])) {
                    return false; // Duplicate in column
                }
                // Check the 3x3 sub-grid
                int subGridRow = 3 * (i / 3) + j / 3;
                int subGridCol = 3 * (i % 3) + j % 3;
                if (board[subGridRow][subGridCol] != '.' && !subGridSet.add(board[subGridRow][subGridCol])) {
                    return false; // Duplicate in sub-grid
                }
            }
        }
        return true; // If no duplicates were found, return true
    }
}