Valid Sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules (http://sudoku.com.au/TheRules.aspx).
The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

Solution: 1. Traverse the Sudoku only once.
2. Bit manipulation. Use only one bit to represent a number. Space: sizeof(int) * (1+9+9).

class Solution {
public:
    bool isValidSudoku(vector<vector<char> > &board) {
        const int N = 9;
        int col[N] = {0};
        int box[N] = {0};
        
        for(int i = 0; i < N; i++) {
            int row = 0;
            for(int j = 0; j < N; j++) {
                if(board[i][j] == '.') {
                    continue;
                }
                int bit = 1 << (board[i][j] - '1');
                int box_index = i/3*3 + j/3;
                if((bit & row) || (bit & col[j]) || (bit & box[box_index])) {
                    return false;
                }
                row |= bit;
                col[j] |= bit;
                box[box_index] |= bit;
            }
        }
        return true;
    }
};