题目：LeetCode 005 Longest Palindromic Substring
Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.
题意：字符串S，找到S中的最长回文子串。假定S最长为1000，并且有且仅有一个最长回文子串。