36. 有效的数独

题目：判断一个 9x9 的数独是否有效。只需要根据以下规则，验证已经填入的数字是否有效即可。
数字 1-9 在每一行只能出现一次。
数字 1-9 在每一列只能出现一次。
数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。

示例 1:
输入:
[
["5","3",".",".","7",".",".",".","."],
["6",".",".","1","9","5",".",".","."],
[".","9","8",".",".",".",".","6","."],
["8",".",".",".","6",".",".",".","3"],
["4",".",".","8",".","3",".",".","1"],
["7",".",".",".","2",".",".",".","6"],
[".","6",".",".",".",".","2","8","."],
[".",".",".","4","1","9",".",".","5"],
[".",".",".",".","8",".",".","7","9"]
]
输出: true

代码：
class Solution {
public boolean isValidSudoku(char[][] board) {
var row_set=new HashSet(); //利用set集合快速查看重复元素，也可以利用两个二维数组，分别记录第i行/第j列中第几个元素是否已存在，
var col_set=new HashSet(); //如第i行row_set[i][j]，代表第i行中数字为j+1（下标从0开始）的数是否存在。
boolean[][] block=new boolean[9][9];
int blockIndex=0;
for(int i=0;i<9;i++){ //行是否满足
for(int j=0;j<9;j++){
if(board[i][j]!='.'){
if(row_set.contains(board[i][j])){return false;}
else{row_set.add(board[i][j]);}
}
if(board[j][i]!='.') { //列是否满足
if(col_set.contains(board[j][i])){return false;}
else{col_set.add(board[j][i]);}
}
if(board[i][j]!='.'){
blockIndex=i/3*3+j/3; //块是否满足 这一步很重要，我没想到，怎么通过行列转换成第几块。
int num=board[i][j] - '0';
if(block[blockIndex][num-1]==false){
block[blockIndex][num-1]=true;
}
else{return false;}
}
}
row_set.clear();
col_set.clear();
}
return true;

}

}