Three flags are used to check whether a number appear.

used1: check each row

used2: check each column

used3: check each sub-boxes

```
class Solution
{
public:
bool isValidSudoku(vector<vector<char> > &board)
{
int used1[9][9] = {0}, used2[9][9] = {0}, used3[9][9] = {0};
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, k = i / 3 * 3 + j / 3;
if(used1[i][num] || used2[j][num] || used3[k][num])
return false;
used1[i][num] = used2[j][num] = used3[k][num] = 1;
}
return true;
}
};
```