f=lambda s:s and f(s[:-1])+s[-1]*(s.count(s[-1])<3)
for s in '', 'xxxxx', 'abcabc', 'abcdabcaba', 'abacbadcba', 'aaabcbccdbabdcd':
print s
print f(s)
Zj1sYW1iZGEgczpzIGFuZCBmKHNbOi0xXSkrc1stMV0qKHMuY291bnQoc1stMV0pPDMpCgpmb3IgcyBpbiAnJywgJ3h4eHh4JywgJ2FiY2FiYycsICdhYmNkYWJjYWJhJywgJ2FiYWNiYWRjYmEnLCAnYWFhYmNiY2NkYmFiZGNkJzoKCXByaW50IHMKCXByaW50IGYocyk=