LeetCode 5: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.

分析：

采用中心扩展法，每个字符都有可能是回文字符串的中点。依次求得各字符作为中心的回文字符串，取其中最长的那个即可。

注意到回文字符串分为奇、偶两种类型对应“aba”和“aa”，所以每个字符都要求两次回文字符。

代码如下：

class Solution {
public:
    string str_helper(string s, int left, int right)
    {
        int n = s.length();
        while(left>=0 && right<=n-1 && s[left]==s[right]){
            left--;
            right++;
        }
        return s.substr(left+1, right-left-1);
        
    }
    string longestPalindrome(string s) {
            if(s.length() <= 1)
                return s;
        string longest = s.substr(0,1);
        for (int i=0; i<s.length(); i++){
            
            string tmp = str_helper(s, i, i);
            if (tmp.length() > longest.length())  {
                longest = tmp;
            }
            tmp = str_helper(s, i, i+1);
            if (tmp.length() > longest.length()) {
                longest = tmp;
            }
        }
    
        return longest;
    }

};