SpiralMatrixII.java

public int[][] generateMatrix(int n) {
    assert n >= 0;

    int[][] res = new int[n][n];

    this.generateMatrixHelper(0, 0, n, n, res, 1);
    return res;
}

private void generateMatrixHelper(
        int i,
        int j,
        int rowSize,
        int colSize,
        int[][] matrix,
        int num
) {
    if (rowSize <= 0 || colSize <= 0) {
        return;
    }

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

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

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

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

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

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

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