* 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.

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

public class Solution {
public boolean isValidSudoku(char[][] board) {
    for (int i=0; i<9; i++) {
        if (!isParticallyValid(board,i,0,i,8)) return false;
        if (!isParticallyValid(board,0,i,8,i)) return false;
    }
    for (int i=0;i<3;i++){
        for(int j=0;j<3;j++){
            if (!isParticallyValid(board,i*3,j*3,i*3+2,j*3+2)) return false;
        }
    }
    return true;
}
private boolean isParticallyValid(char[][] board, int x1, int y1,int x2,int y2){
    Set singleSet = new HashSet();
    for (int i= x1; i<=x2; i++){
        for (int j=y1;j<=y2; j++){
            if (board[i][j]!='.') if(!singleSet.add(board[i][j])) return false; // If this set already contains the element, the call leaves the set unchanged and returns false.
        }
    }
    return true;
}

}

https://leetcode.com/discuss/17990/sharing-my-easy-understand-java-solution-using-set