Prints a spiral ordering of a 2d matrix in O(N) time and O(1) space.

print_spiral_matrix.py

def spiralOrder(matrix):
    """
    >>> spiralOrder([[1,2,3],[4,5,6],[7,8,9]])
    1
    2
    3
    6
    9
    8
    7
    4
    5
    """
    def spiral_coords(r1, c1, r2, c2):
        for c in range(c1, c2 + 1):
            yield r1, c
        for r in range(r1 + 1, r2 + 1):
            yield r, c2
        if r1 < r2 and c1 < c2:
            for c in range(c2 - 1, c1, -1):
                yield r2, c
            for r in range(r2, r1, -1):
                yield r, c1

    if not matrix:
        return
    r1, r2 = 0, len(matrix) - 1
    c1, c2 = 0, len(matrix[0]) - 1
    while r1 <= r2 and c1 <= c2:
        for r, c in spiral_coords(r1, c1, r2, c2):
            print(matrix[r][c])
        r1 += 1; r2 -= 1
        c1 += 1; c2 -= 1