[LeetCode] 36. 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.

// From Discuss
class Solution {
public:
    bool isValidSudoku(vector<vector<char>>& board) {
        //used1: check each row
        //used2: check each column
        //used3: check each sub-boxes
        bool used1[9][9]={false}, used2[9][9]={false}, used3[9][9]={false};
        
        for (int i = 0; i < board.size(); ++i){
            for (int j = 0; j < board[i].size(); ++j){
                if (board[i][j] != '.'){
                    int num = board[i][j] - '0' - 1;
                    int k = i / 3 * 3 + j / 3;
                    if (used1[i][num] || used2[j][num] || used3[k][num]){
                        return false;
                    }else{
                        used1[i][num] = used2[j][num] = used3[k][num] = true;
                    }
                }
            }
        }
        
        return true;
    }
};