【LeetCode】36. Valid Sudoku

题目：

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.

提示：

此题最主要的难点其实是如何记录每一块中数字出现的情况（每行和每列比较简单）。

如果我们以从左到右，从上到下的顺序给每一块标定次序的话，那个对于第i行j列的元素，它隶属于i / 3 * 3 + j / 3。

这个问题解决之后，其他问题也就比较简单了。

代码：

class Solution {
public:
    bool isValidSudoku(vector<vector<char>>& board) {
        int mark_row[9][9] = {0}, mark_col[9][9] = {0}, mark_block[9][9] = {0};
        for (int i = 0; i < 9; ++i) {
            for (int j = 0; j < 9; ++j) {
                if (board[i][j] == '.') continue;
                int idx = board[i][j] - '1', k = i / 3 * 3 + j / 3;
                if (mark_row[i][idx] || mark_col[j][idx] || mark_block[k][idx]) return false;
                else mark_row[i][idx] = mark_col[j][idx] = mark_block[k][idx] = 1;
            }
        }
        return true;
    }
};