leetcode - 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.

class Solution {
public:
    bool isValidSudoku(std::vector<std::vector<char> > &board) {
        int rows = board.size();
        int cols = board[0].size();
        int mask=0;
        int val;
        for(int i=0;i<rows;i++){
            mask=0;
            for(int j=0;j<cols;j++){
                if(board[i][j]!='.'){
                    int val = (int)(board[i][j]-'0');
                    if(mask & (1<<val))
                        return false;
                    else mask |= (1<<val);
                }
            }
        }
        //rows are checked
        for(int j=0;j<cols;j++){
            mask=0;
            for(int i=0;i<rows;i++){
                if(board[i][j]!='.'){
                    int val = (int)(board[i][j]-'0');
                    if(mask & (1<<val))
                        return false;
                    else mask |= (1<<val);
                }
            }
        }
        //cols are checked
        for(int i=0;i<rows;i+=3)
        {
            for(int j=0;j<cols;j+=3)
            {
                mask=0;
                for(int k=i;k<=i+2;k++)
                {
                    for(int l=j;l<=j+2;l++)
                    {
                        if(board[k][l]!='.'){
                            val = (int)(board[k][l]-'0');
                            if(mask&(1<<val))
                                return false;
                            else mask|=(1<<val);
                        }
                    }
                }
            }
        }
        return true;
    }
};