顺时针打印矩阵

public int[] spiralOrder(int[][] matrix) {
    if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
        return new int[0];
    }
    int rows = matrix.length;
    int columns = matrix[0].length;
    // 判断路径是否被访问过
    boolean[][] visited = new boolean[rows][columns];
    int total = rows * columns;
    // 总路径长度为矩阵的元素数量
    int[] order = new int[total];
    int row = 0;
    int column = 0;
    // 移动的方向
    int[][] directions = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
    int directionIndex = 0;
    for (int i = 0; i < total; i++) {
        order[i] = matrix[row][column];
        visited[row][column] = true;
        int nextRow = row + directions[directionIndex][0];
        int nextColumn = column + directions[directionIndex][1];
        if (nextRow < 0 || nextRow >= rows || nextColumn < 0 || nextColumn >= columns
                || visited[nextRow][nextColumn]) {
            directionIndex = (directionIndex + 1) % 4;
        }
        row += directions[directionIndex][0];
        column += directions[directionIndex][1];
    }
    return order;
}