36. Valid Sudoku 

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class Solution {
    public boolean isValidSudoku(char[][] board) {
        boolean[] appear = new boolean[9];

        for (int i = 0; i < 9; i++) {
            Arrays.fill(appear, false);
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.') {
                    continue;
                }

                if (appear[board[i][j] - '1']) {
                    return false;
                } else {
                    appear[board[i][j] - '1'] = true;
                }
            }
        }

        for (int j = 0; j < 9; j++) {
            Arrays.fill(appear, false);
            for (int i = 0; i < 9; i++) {
                if (board[i][j] == '.') {
                    continue;
                }

                if (appear[board[i][j] - '1']) {
                    return false;
                } else {
                    appear[board[i][j] - '1'] = true;
                }
            }
        }

        for (int i = 0; i <= 6; i += 3) {
            for (int j = 0; j <= 6; j += 3) {
                Arrays.fill(appear, false);
                for (int k = i; k < i + 3; k++) {
                    for (int l = j; l < j + 3; l++) {
                        if (board[k][l] == '.') {
                            continue;
                        }

                        if (appear[board[k][l] - '1']) {
                            return false;
                        } else {
                            appear[board[k][l] - '1'] = true;
                        }
                    }
                }
            }
        }

        return true;
    }
}
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class Solution {
    public boolean isValidSudoku(char[][] board) {
        int[][] rows = new int[9][9];
        int[][] cols = new int[9][9];
        int[][][] boxes = new int[3][3][9];

        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] != '.') {
                    int index = board[i][j] - '1';

                    rows[i][index]++;
                    cols[j][index]++;
                    boxes[i / 3][j / 3][index]++;

                    if (rows[i][index] > 1 || cols[j][index] > 1 || boxes[i / 3][j / 3][index] > 1) {
                        return false;
                    }
                }
            }
        }

        return true;
    }
}

References

36. Valid Sudoku