LeetCode - Valid Sudoku

public class Solution {
   public boolean isValidSudoku(char[][] board) {
       
		StringBuilder str = new StringBuilder("");
		 //row
		for(int i=0; i<9; i++) {
			for(int j=0; j<9; j++) {
				if(board[i][j] != ‘.‘) {
					str.append(board[i][j]);
				}
			}
			if(isOnce(str.toString())) {
				str.delete(0, str.length());
			} else {
				return false;
			}
		}
		
		//col
		
		for(int i=0; i<9; i++) {
			for(int j=0; j<9; j++) {
				if(board[j][i] != ‘.‘) {
					str.append(board[j][i]);
				}
			}
			if(isOnce(str.toString())) {
				str.delete(0, str.length());
			} else {
				return false;
			}
		}
		
		//3X3
		for(int cubeX=0; cubeX<=6; cubeX+=3) {
			for(int cubeY=0; cubeY<=6; cubeY+=3) {
				if(!isCubeOnce(board, cubeX, cubeY)) {
					return false;
				}
			}
		}
		
		return true;
		
    }
	public static boolean isCubeOnce(char[][] chArr, int i, int j) {
		StringBuilder str = new StringBuilder("");
		for(int ii=i; ii<i+3; ii++) {
			for(int jj=j; jj<j+3; jj++) {
				if(chArr[ii][jj] != ‘.‘)
					str.append(chArr[ii][jj]);
			}
		}
		if(isOnce(str.toString())) {
			return true;
		} 
		else {
			return false;
		}
	}
	public static boolean isOnce(String s) {
		java.util.Set<Character> set = new java.util.HashSet<>();
		for(int i=0; i<s.length(); i++) {
			set.add(s.charAt(i));
		}
		return set.size()==s.length();
	}
}