python 面试题判断一个字符串是否是回响字符串 abbbba

a='abbba'
num=len(a)
if num%2==0:
    begin=0
    end=len(a)-1
    if a[begin] == a[end]:
        print('yes')
    else:
        print('not')
else:
    mid=num//2+1
    a.replace('%s'%a[mid],'')
    begin = 0
    end = len(a) - 1
    if a[begin] == a[end]:
        print('yes')
    else:
        print('not')