需求: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. 如果一个字符串从左向右写和从右向左写是一样的,这样的字符串就叫做palindromic string 判断回文数,中间开花.定一个中心,向两边散