Leetcode 336. Palindrome Pairs

文章作者：Tyan
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1. Description

Palindrome Pairs

2. Solution

解析：Version 1简单，容易理解，但超时。Version 2是将字符串分为两部分，前半部分和后半部分，如果两部分有一部分是回文子串，则寻找另一部分的对应的回文字符串。

• Version 1

class Solution:
    def palindromePairs(self, words):
        result = []
        length = len(words)
        for i in range(length):
            for j in range(i + 1, length):
                positive = words[i] + words[j]
                reverse = words[j] + words[i]
                if self.checkPalindrome(positive):
                    result.append([i, j])
                if self.checkPalindrome(reverse):
                    result.append([j, i])
        return result


    def checkPalindrome(self, word):
        i = 0
        j = len(word) - 1
        while i < j:
            if word[i] != word[j]:
                return False
            i += 1
            j -= 1
        return True

• Version 2

class Solution:
    def palindromePairs(self, words):
        result = []
        stat = {}
        for index, word in enumerate(words):
            stat[word] = index

        for index, word in enumerate(words):
            length = len(word)
            if length == 1 and '' in stat:
                result.append([index, stat['']])
                result.append([stat[''], index])
                continue
            for i in range(length + 1):
                prefix = word[:i]
                rest = word[i:]
                reverse = rest[::-1]
                if self.checkPalindrome(prefix):
                    if reverse in stat and index != stat[reverse]:
                        res = [stat[reverse], index]
                        if res not in result:
                            result.append(res)

                suffix = word[i:]
                rest = word[:i]
                reverse = rest[::-1]
                if self.checkPalindrome(suffix):
                    
                    if reverse in stat and index != stat[reverse]:
                        res = [index, stat[reverse]]
                        if res not in result:
                            result.append(res)

        return result


    def checkPalindrome(self, word):
        i = 0
        j = len(word) - 1
        while i < j:
            if word[i] != word[j]:
                return False
            i += 1
            j -= 1
        return True

Reference

1. https://leetcode.com/problems/palindrome-pairs/