各位牛顿，你们做这个需要多久算出来？

class Solution {
public void solveSudoku(char[][] board) {
dfs(board, 0);
}

boolean check(int i, int j, char[][] board, int target) {
for (int k=0; k<9; k++) {if (board[i][k] - '0' == target) {return false;}}
for (int k=0; k<9; k++) {if (board[k][j] - '0' == target) {return false;}}
for (int a=0; a<3; a++) {
for (int b=0; b<3; b++) {if (board[(i/3)*3+a][(j/3)*3+b] - '0' == target) {return false;}}
}
return true;
}

boolean dfs(char[][] board, int index) {
if (index < 81) {
int i = index/9; int j = index%9;
if (board[i][j] != '.') {return dfs(board, index+1);}
for (int k=1; k<10; k++) {
if (check(i, j, board, k)) {
board[i][j] = (char) (k+'0');
if (dfs(board, index+1)) {return true;}
else {board[i][j] = '.';}
}
}
return false;
}
return true;
}
}