回文是指正着读和倒着读，结果一些样，比如abcba或abba。

听说有三种方法：

法一：就是我这种呆子能想到的，遍历啊，但时间复杂度应该是O^3，而且算法过程复杂，要是没我的思路清晰，头脑敏捷，不骄不躁，就换下一个方法吧O(∩_∩)O

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.util.ArrayList;

import java.util.Scanner;

public class Five{

public static void longestPalindrome(String s) {

ArrayList<Integer> List = new ArrayList<Integer>();

String temp="0";

for(int i=0;i<s.length();i++)

{

char item1=s.charAt(i);

temp=temp+item1;

for(int j=i+1;j<s.length();j++)

if(s.charAt(j)!=item1)

temp=temp+s.charAt(j);

else

{

temp=temp+s.charAt(j);

int loc = temp.indexOf("0");

String newStr=temp.substring(0,loc);

List.add(newStr);

}

}

String LongestSubstring = null;

for(int k=1;k<List.size();k++)

{

System.out.println((String) List.get(k-1));

if(((String) List.get(k-1)).length()<((String) List.get(k)).length())

LongestSubstring=((String) List.get(k));

else

LongestSubstring=((String) List.get(k-1));

}

System.out.println(LongestSubstring);

}

public static void main(String args[]) throws IOException{

//BufferedReader rd = new BufferedReader(new InputStreamReader(System.in));

System.out.println("请输入一个字符串：");

Scanner scanner = new Scanner(System.in);

String line = scanner.nextLine();

longestPalindrome(line);

}

}

法二：动态规划。。参考别人的，自己肯定是想不到啦

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.util.ArrayList;

import java.util.Scanner;

public class Five_third{

public static String longestPalindrome(String s) {

if (s == null || s.length() == 0) return "";

int n = s.length();

int max = 0, start = 0, end = 0;

boolean[][] c = new boolean[n][n];

for (int i = 0; i < n; i++) {

for (int j = 0; j < n; j++) {

c[i][j] = i >= j ? true : false;

}

}

//c[i][j] 

for (int j = 1; j < n; j++) {

for (int i = 0; i < j; i++) {

if (s.charAt(i) == s.charAt(j) && c[i+1][j-1]) {

c[i][j] = true;

if (j - i + 1 > max) {

max = j - i + 1;

start = i;

end = j;

}

}

else

c[i][j] = false;

}

}

return s.substring(start, end+1);

}

public static void main(String[] args){

System.out.println("请输入一个字符串：");

Scanner scanner = new Scanner(System.in);

String line = scanner.nextLine();

String ins=longestPalindrome(line);

System.out.println(ins);

}

}