LeetCode 刷题 ---39解数独

36. 有效的数独

class Solution {
    // 行row[i][digit]:每i行存在的数字是digit
    private boolean[][] row = new boolean[9][9];
    // 列column[j][digit]:每j列存在的数字是digit
    private boolean[][] column = new boolean[9][9];
    // 块block[i][j][digit]:3x3块中(i, j)存在的数字是digit
    private boolean[][][] block = new boolean[3][3][9];
    // valid: 是否符合条件
    private boolean valid = false;
    // 每个空格的位置
    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; // 0 - 8
                    row[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;
        }

        int[] space = spaces.get(pos);
        int i = space[0];
        int j = space[1];
        for (int digit = 0; digit < 9 && !valid; digit++){
            if (!row[i][digit] && !column[j][digit] && !block[i / 3][j / 3][digit]){
                row[i][digit] = column[j][digit] = block[i/3][j/3][digit] = true;
                board[i][j] = (char)(digit + '0' + 1); // 1 - 9
                dfs(board, pos + 1);
                row[i][digit] = column[j][digit] = block[i/3][j/3][digit] = false;
            }
        }

    }
}