LeetCode——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.

给定一个字符串S，找出其中的最长回文字符子串。

1.枚举所有子串，并判断是否是回文串同时记录最大长度。超时。

//找出所有子串，并判断是否是回文串，每次记录长度public static String longestPalindrome1(String s) {String ret = "";int maxlen = 0;for(int i=0;i<s.length();i++){for(int j=i+1;j<s.length();j++){String temp = s.substring(i,j+1);int len = j - i;if(isPalindrome(temp)){if(len > maxlen){ret = temp;maxlen = len;}}}}return ret;}public static boolean isPalindrome(String s){for(int i=0;i<s.length();i++){if(s.charAt(i) != s.charAt(s.length() - i - 1))return false;}return true;}

2.由某个中心向两侧扩展寻找，找到可以扩展的最大长度。

public static String longestPalindrome(String s) {if (s.length() <= 1) return s;String longest = s.substring(0, 1);for (int i = 0; i < s.length(); i++) {String tmp = helper(s, i, i);if (tmp.length() > longest.length()) {longest = tmp;}tmp = helper(s, i, i + 1);if (tmp.length() > longest.length()) {longest = tmp;}}return longest;}public static String helper(String s, int begin, int end) {while (begin >= 0 && end <= s.length() - 1 && s.charAt(begin) == s.charAt(end)) {begin--;end++;}return s.substring(begin + 1, end);}