20 lines
610 B
Python
20 lines
610 B
Python
# https://leetcode.com/problems/unique-length-3-palindromic-subsequences
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class Solution:
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def countPalindromicSubsequence(self, s: str) -> int:
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first, last = {}, {}
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for i in range(len(s)):
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if s[i] not in first:
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first[s[i]] = i
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last[s[i]] = i
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palinedromes = set()
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for ch in first:
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i = first[ch]
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j = last[ch]
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if i + 1 < j:
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middles = s[i + 1 : j]
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for middle in middles:
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palinedromes.add(''.join([ch, middle, ch]))
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return len(palinedromes)
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