【LeetCode OJ】Longest Palindromic Substring

题目链接：https://leetcode.com/problems/longest-palindromic-substring/

题目：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.

解题思路：palindromic-substring是指回文串，例如abba、abcba，它有一个特点就是从字符串的中间开始往两边的字符都是一样的。我们可以从字符串的第二个字符开始向左边和右边同时扫描，找到字符长度最长的回文串。示例代码如下：

public class Solution 
{
    public String longestPalindrome(String s)
    {
          int n = s.length();
          if (n <= 1)
              return s;
         int maxlen = 1, k, j, a = 0;
         int l;
         for (int i = 1; i < n;)
         {
            k = i - 1;
            j = i + 1;
            //扫描左边与s[i]相同的字符
            while (k >= 0 && s.charAt(k) == s.charAt(i))
                  k--;
            //扫描右边与是s[i]相同的字符
            while (j < n && s.charAt(j) == s.charAt(i))
                    j++;
            while (k >= 0 && j < n && s.charAt(k) == s.charAt(j))
            {
                k--;
                j++;
            }
            l = j - k - 1;
            if (maxlen < l) 
            {
                a = k + 1;
                maxlen = l;
            }
            i++;
         }
          return s.substring(a, a+maxlen);
    }