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.

Example:

Input: "babad"

Output: "bab"

Note: "aba" is also a valid answer.
Example:

Input: "cbbd"

Output: "bb"

今天先上一种解法，递归：

class Solution {
public:
    string longestPalindrome(string s) {
        if(s.empty())
            return 0;
        int start_pos = 0, max_len = 1;  //start_pos default 0 because max_len default 1
        const int len = s.length();
        for(int i=0; i<len-1; ++i){
            extend_palindrome(s, len, i, i, start_pos, max_len);  //assume odd length, try to extend palindrome as possible
            extend_palindrome(s, len, i, i+1, start_pos, max_len);   //assume even length
        }
        return s.substr(start_pos, max_len);
    }
    void extend_palindrome(string& s, int len, int left, int right, int& start_pos, int& max_len){
        while(left >= 0 && right < len && s[left] == s[right]){
            --left;
            ++right;
        }

        if(max_len < right-left-1){
            start_pos = left + 1;
            max_len = right - left - 1;
        }
    }
};

为什么要有两种情况呢：比如abba和abab分别就是和偶数个最长和奇数个最长的情况。