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class Solution {
private boolean[][] line = new boolean[9][9];
private boolean[][] column = new boolean[9][9];
private boolean[][][] block = new boolean[3][3][9];
private boolean valid = false;
private List<int[]> spaces = new ArrayList<int[]>();
public void solveSudoku(char[][] board) {
for (int i = 0; i < 9; ++i) {
for (int j = 0; j < 9; ++j) {
if (board[i][j] == '.') {
spaces.add(new int[]{i, j});
} else {
int digit = board[i][j] - '0' - 1;
在i行，j列，第block[i / 3][j / 3]中出现过digit+1
line[i][digit] = column[j][digit] = block[i / 3][j / 3][digit] = true;
}
}
}
dfs(board, 0);
}
public void dfs(char[][] board, int pos) {
if (pos == spaces.size()) {
valid = true;
return;
}
获取第pos个空白位置
int[] space = spaces.get(pos);
空白位置的行，列
int i = space[0], j = space[1];
遍历1~9数字，哪个未放入，放入后继续进行递归下个空白位置，直到全部空格填完，进行回溯;
或者1~9都不能放入这个空白位置，回溯
for (int digit = 0; digit < 9 && !valid; ++digit) {
if (!line[i][digit] && !column[j][digit] && !block[i / 3][j / 3][digit]) {
line[i][digit] = column[j][digit] = block[i / 3][j / 3][digit] = true;
board[i][j] = (char) (digit + '0' + 1);
dfs(board, pos + 1);
line[i][digit] = column[j][digit] = block[i / 3][j / 3][digit] = false;
}
}
}
}
