Longest Palindromic Substring - 字符串中最长的回文字段

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，向两边散开。这个中心是从左到右移动的。需要2个游标。

int palindrome(String ps,int left,int right) 这个方法用来判断回文字段，返回字段长度

String longestPalindrome(String s) 在这里调用palindrome方法，遍历字符串去找。遍历过程注意不要越界。

以下为Java代码：

 /**
  * @author Rust Fisher
  * @date 2015-9-14
  */
 public class LongestPalindromicSubstring {
     /**
      * @param s - input string
      * @return the Longest Palindromic Substring
      */
     public static String longestPalindrome(String s) {
         String result = "";
         int inputLenght = s.length();
         int startIndex = 0;
         int longest = 1;

         for (int i = 0; i < inputLenght; i++) {
             int oddLen = 1,dualLen = 1, currentLen;
             oddLen = palindrome(s, i, i);
             if (i+1 < inputLenght) {
                 dualLen = palindrome(s, i, i+1);
             }
             currentLen = dualLen > oddLen ? dualLen : oddLen;
             if (currentLen > longest){
                 longest = currentLen;
                 if (longest%2 == 0) {
                     startIndex = i - longest/2 + 1;
                 } else {
                     startIndex = i - longest/2;
                 }
             }
         }
         for (int i = startIndex; i < startIndex+longest; i++) {
             result += s.charAt(i);
         }
         return result;
     }
     /**
      * @param ps - input string
      * @param left - index move left
      * @param right - index move right
      * @return the current length of palindrome
      */
     public static int palindrome(String ps,int left,int right){
         int thislen = 0;
         int len = ps.length();
         while(left >= 0 && right < len && ps.charAt(left) == ps.charAt(right)){
             left--;
             right++;
         }
         thislen = right - left - 1;
         return thislen;
     }
     public static void main(String args[]){
         System.out.println(longestPalindrome("hfiwafhaabbaaccddio128213"));
         System.out.println(longestPalindrome("abcdefgfedcba"));
         System.out.println(longestPalindrome("abc"));
     }
 }

输出：

aabbaa
abcdefgfedcba
a