python代码：为什么我的代码不返回？

Question

在函数LCSlist(i,j)的末尾，它应该返回['a', 't', 't', 'a', 'c', 'g', 'c', 't', 'a']。但我的输出是＆＃34;无＆＃34;。当我检查输出的类型时，类是＆ ＃34; NoneType＆＃34; （<class 'NoneType'>）。有人可以帮忙吗？

这是我的代码：

s1 = 'aatcatacggcatac'
s2 = 'cattaccagactag'

ls1 = len(s1)
ls2 = len(s2)

DP = [[-1 for i in range(ls2+1)] for j in range(ls1+1)]
for i in range(ls1):
    DP[i][0] = 0
for j in range(ls2):
    DP[0][j] = 0
#print(DP)

track = [[-1 for i in range(ls2+1)] for j in range(ls1+1)]


def LCS(i,j):
    #print(DP)
    if DP[i][j] != -1:
        return DP[i][j]

    else:
        if(s1[i-1] == s2[j-1]):
            track[i-1][j-1] = 3
            DP[i][j] = 1 + LCS(i-1,j-1)

        else:
            r1 = LCS(i-1,j)
            r2 = LCS(i,j-1)
            DP[i][j] = max(r1 , r2)

            if DP[i][j] == r1:          
                track[i-1][j-1] = 1
            else:
                track[i-1][j-1] = 2

        return DP[i][j]


alist = []
def LCSlist(i,j):
    #print(alist)
    if (i < 0) | (j < 0):
        alist.reverse()
        print(alist)
        return alist

    else:
        if track[i][j] == 3:
            alist.append(s2[j])
            print(alist)
            LCSlist(i-1,j-1)
        elif track[i][j] == 2:
            LCSlist(i,j-1)
        else:
            LCSlist(i-1,j)


print( "Ans is",LCS(ls1,ls2) )
print( "LCSlist is" ,type(LCSlist(ls1-1,ls2-1)))