leetcode 336. 回文对

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给定一组独特的单词， 找出在给定列表中不同 的索引对(i, j),使得关联的两个单词，例如：words[i] + words[j]形成回文。

示例 1:
给定 words = ["bat", "tab", "cat"]
返回 [[0, 1], [1, 0]]
回文是 ["battab", "tabbat"]

示例 2:
给定 words = ["abcd", "dcba", "lls", "s", "sssll"]
返回 [[0, 1], [1, 0], [3, 2], [2, 4]]
回文是 ["dcbaabcd", "abcddcba", "slls", "llssssll"]` 

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class Solution(object):
    def palindromePairs(self, words):
        """
        :type words: List[str]
        :rtype: List[List[int]]
        """
        def is_reverse(word):
            return word==word[::-1]
        
        words = {word:k for k,word in enumerate(words)}
        res = []
        for word,k in words.iteritems():
            n = len(word)
            for i in range(n+1):
                pre = word[:i]
                suf = word[i:]
                if is_reverse(pre):
                    back = suf[::-1]
                    if back != word and back in words:
                        res.append([words[back],k])
                if i != n and is_reverse(suf):
                    back = pre[::-1]
                    if back != word and back in words:
                        res.append([k,words[back]])
        return res