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"

本题无他法，唯有蛮力而已。注意怎样组织代码，里面的门道很多。在这个上面吃了很多亏。
另外要注意的是，本题中或者类似的题，判断是不是palindromic substring的方法都是从字符串的中间出发，向两端扩展，判断字符是否对称。

class Solution {
public:
    string longestPalindrome(string s) {
        int n = s.size();
        if (n <= 1)
            return s;
            
        int maxlen = 0,
            bestStart = 0;
        for (int i = 0; i <= n-1; ++i) {
            int curlen = 0;
            if (i <= n - 2) {
                if (s[i] == s[i+1]) {
                    curlen = isPalindrome(s, i, i+1);
                }
                if (curlen > maxlen) {
                    maxlen = curlen;
                    bestStart = i + 1 - maxlen/2;
                }
            }
            curlen = isPalindrome(s, i, i);
            if (curlen > maxlen) {
                maxlen = curlen;
                bestStart = i - (maxlen - 1) / 2;
            }
        }
        return s.substr(bestStart, maxlen);
    }
private:
    int isPalindrome(const string& s, int pos1, int pos2) {
        int n = s.size();
        while (pos1 >= 1 && pos2 <= n-2) {
            if (s[pos1 - 1] == s[pos2 + 1]) {
                --pos1;
                ++pos2;
            } else {
                break;
            }
        }
        int len = pos2 - pos1 + 1;
        return len;
    }
};