Unique Length-3 Palindromic Subsequences.py

class Solution(object):
    def countPalindromicSubsequence(self, s):
        d=defaultdict(list)
        for i,c in enumerate(s):
            d[c].append(i)
        ans=0
        for el in d:
            if len(d[el])<2:
                continue
            a=d[el][0]
            b=d[el][-1]
            ans+=len(set(s[a+1:b]))
        return(ans)