问题

给定一个字符串s，找到s中最长的回文子串。
示例1

输入: "babd"
输出: "bab"

示例2

输入: "cbbd"
输出: "bb"

代码

public class LongestPalindromicSubstring {
    public static void main(String[] args) {
        String s="babd";
        System.out.println(longestPalindrome(s));
    }
    public static String longestPalindrome(String s) {
        if (s==null || s.length()==0){
            return "";
        }else{
            int leftIndex=0,rightIndex=0;
            for (int i=0;i<s.length();i++){
                int len1=expandAroundCenter(i,i,s);
                int len2=expandAroundCenter(i,i+1,s);
                int maxLen=Math.max(len1,len2);
                if (maxLen>rightIndex-leftIndex){
                    leftIndex=i-(maxLen-1)/2;
                    rightIndex=i+maxLen/2;
                }
            }
            return s.substring(leftIndex,rightIndex+1);
        }
    }
    public static int expandAroundCenter(int leftIndex,int rightIndex,String s){
        while(leftIndex>=0 && rightIndex<s.length() && (s.substring(leftIndex,leftIndex+1)).equals(s.substring(rightIndex,rightIndex+1))){
            leftIndex--;
            rightIndex++;
        }
        return rightIndex-leftIndex-1;
    }
}