35. Valid Sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

A partially filled sudoku which is valid.

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public class Solution {

    public boolean isValidSudoku(char[][] board) {
        int n = board.length;

        // check each row
        for (int i = 0; i < n; i++) {
            if (!checkRow(i, board, n))
                return false;
        }
        
        // check each col
        for (int j = 0; j < n; j++) {
            if (!checkCol(j, board, n))
                return false;
        }
        
        // check 9*9 block
        for (int i = 0; i < n; i=i+3) {
            for (int j = 0; j<n; j=j+3) {
                if (!checkBlock(i, j, board, n))
                    return false;
            }
        }

        return true;
    }

    public boolean checkBlock(int r, int c, char[][] board, int n) {
        
        int helper = 0;
        
        for (int i = r; i < r+3; i++) {
            for (int j = c; j < c+3; j++) {
                int val = board[i][j];
                if( (helper & (1<<val)) > 0)
                    return false;
                if (val != '.')
                    helper |= 1 << val;
            }
        }
        return true;
    }

    public boolean checkRow(int r, char[][] board, int n) {

        int helper = 0;
        for (int j = 0; j < n; j++) {
            int val = board[r][j];
            if( (helper & (1<<val)) > 0)
                return false;
            if (val != '.')
                helper |= 1 << val;
        }
        return true;
    }

    public boolean checkCol(int c, char[][] board, int n) {
        int helper = 0;
        for (int i = 0; i < n; i++) {
            int val = board[i][c];
            if( (helper & (1<<val)) > 0)
                return false;
            if (val != '.')
                helper |= 1 << val;
        }
        return true;
    }
}