SpiralMatrix.java

public List<Integer> spiralOrder(int[][] matrix) {
    List<Integer> res = new ArrayList<>();

    if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
        return res;
    }

    this.printSpiralOrder(0, 0, matrix.length, matrix[0].length, res, matrix);

    return res;
}

// printSpiralOrder is a poor name, function starts with verb, printSpiralOrder, class is noun, function is verb
private void printSpiralOrder(int i, int j, int rowSize, int colSize, List<Integer> res, int[][] matrix) {
    if (rowSize <= 0 || colSize <= 0) {
        return;
    }

    if (rowSize == 1 && colSize == 1) {
        res.add(matrix[i][j]);
        return;
    }
    if (rowSize == 1) {
        for (int k = j; k < j + colSize; k++) {
            res.add(matrix[i][k]);
        }
        return;
    }

    if (colSize == 1) {
        for (int k = i; k < i + rowSize; k++) {
            res.add(matrix[k][j]);
        }
        return;
    }

    // do the spiral
    for (int k = j; k < j + colSize; k++) {
        res.add(matrix[i][k]);
    }

    for (int k = i + 1; k < i + rowSize; k++) {
        res.add(matrix[k][j + colSize - 1]);
    }

    for (int k = j + colSize - 2; k >= i; k--) {
        res.add(matrix[i + rowSize - 1][k]);
    }

    for (int k = i + rowSize - 2; k > i; k--) {   // both the start and end need to be i, j, and also care about length
        res.add(matrix[k][j]);
    }

    this.printSpiralOrder(i + 1, j + 1, rowSize - 2, colSize - 2, res, matrix);
}