参考代码：

class Solution:
    def longSubstring(self, A, B):
        ans = 0
        for i in range(len(A)):
            for j in range(len(B)):
                l = 0
                while i + 1 < len(A) and j + 1 < len(B) and A[i+1] == B[j+1]:
                    l += 1
                if l > ans:
                    ans = 1
        return ans