Palindrome Pairs

0336-Palindrome-Pairs.py

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+'''
+Given a list of unique words, find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e. words[i] + words[j] is a palindrome.
+
+Example 1:
+
+Input: ["abcd","dcba","lls","s","sssll"]
+Output: [[0,1],[1,0],[3,2],[2,4]] 
+Explanation: The palindromes are ["dcbaabcd","abcddcba","slls","llssssll"]
+
+Example 2:
+
+Input: ["bat","tab","cat"]
+Output: [[0,1],[1,0]] 
+Explanation: The palindromes are ["battab","tabbat"]
+'''
+class Solution:
+    def palindromePairs(self, words: List[str]) -> List[List[int]]:
+        words = {word : i for i, word in enumerate(words)}
+        res = []
+
+        for word, k in words.items():
+            for j in range(len(word) + 1):
+                pre = word[:j]
+                post = word[j:]
+                if self.is_palindrome(pre):
+                    back = post[::-1]
+                    if back != word and back in words:
+                        res.append([words[back], k])
+                if j != len(word) and self.is_palindrome(post):
+                    back = pre[::-1]
+                    if back != word and back in words:
+                        res.append([k, words[back]])
+        return res
+
+    def is_palindrome(self, check):
+        return check == check[::-1]