Spiral Matrix

Given a matrix ofmn elements (mrows, n columns), return all elements of the matrix in spiral order.
For example, Given the following matrix:
[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]

You should return [1,2,3,6,9,8,7,4,5].

invariant: 任何时候都对应一个当前有效的上、下、左、右的界。 在当前上、下、左、右界内依次输出每条边，同时更新相应的界，如果上下界或者左右界交错，算法结束。

def spiralOrder(matrix):    res = []    startX, endX, startY, endY = 0, len(matrix[0]) - 1, 0, len(matrix) - 1    while True:        for j in xrange(startX, endX + 1):            res.append(matrix[startY][j])        startY += 1        if startY > endY: break            for i in xrange(startY, endY + 1):            res.append(matrix[i][endX])        endX -= 1        if startX > endX: break            for j in xrange(endX, startX - 1, -1):            res.append(matrix[endY][j])        endY -= 1        if startY > endY: break            for i in xrange(endY, startY - 1, -1):            res.append(matrix[i][startX])        startX += 1        if startX > endX: break    return res;