给定一个字符串，输出最长的回文子串。回文串指的是正的读和反的读是一样的字符串，例如 “aba“，“ccbbcc“。

string='acagdksahbciuaccbbccfdyfavavaaywwusbuyxyxyaodcabgddy'

ans=0#最长子串的长度
s1=''

def prints1():
    global s1,ans
    if len(s1) > ans:
        ans = len(s1)
        print(s1)

for i in range(len(string)):
    for s in string[i:]:
        len_p=len(s1)/2
        le=len(s1)
        if len(s1)%2:
            if s1[:int(len_p)]==s1[int(len_p)+1:][::-1] and le>1:
                prints1()
        else:
            if s1[:int(len_p)]==s1[int(len_p):][::-1] and le>1:
                prints1()
        s1 += s
    s1=''