Python将两个数组合并成一个数组，多维数组变成一维数组

c1 = ["Red","Green","Blue"]
c2 = ["Orange","Yellow","Indigo"]
c1.extend(c2)
c1 == ["Red","Green","Blue","Orange","Yellow","Indigo"]

c1 = ["Red","Green","Blue"]
c2 = ["Orange","Yellow","Indigo"]
c3 = c1 + c2
c3 == ["Red","Green","Blue","Orange","Yellow","Indigo"]

a = [[1,3],[2,4],[3,5]]
a = array(a)
a.flatten()
array([1, 3, 2, 4, 3, 5])