题目：https://leetcode.com/problems/valid-sudoku/
代码(github)：https://github.com/illuz/leetcode
判断一个数独是否有效。
有效的数独不强求有解。
只要同一行、列、块里没有相同数字就行了。
开个数组记录就行了，没什么难度，可以用二进制来表示，表位运算加速。
（注意是判断有效，不是有解，我刚开始给求解了，TLE 了好多次。。。）
English
Because it just checks whether the sudoku is valid, so we can just check the numbers in it only.
But if we check if the sudoku has a solution, then we should use other algorithms. Because a sudoku is VALID doesn't mean that it has a solution.
To check if it has a solution, we can use DFS to check it. But it may take a long time. Because the complexity is O((n*n)!) !
The better algorithm is Dancing Link.