[LeetCode] 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.

用一个指针从0遍历到s.size()-1，从里向外来检测子串是否是回文，分两种情况：1）bab, 2)bb。也就是当前指针可能指向的是奇数回文，也可能是偶数回文。做两次检测。

class Solution {
public:
    string findPal(string &s, int left, int right)
    {
        if (left < 0)
            return s.substr(left+1, 1);
            
        if (right >= s.size())
            return s.substr(right-1, 1);
            
        while(0 <= left && right < s.size())
        {
            if (s[left] != s[right])
                break;
            left--;
            right++;
        }
        
        left++;
        right--;
        
        return s.substr(left, right - left + 1);        
    }
    
    string longestPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (s.size() == 0)
            return "";
            
        string ret;
        
        for(int i = 0; i < s.size(); i++)
        {
            string str = findPal(s, i - 1, i + 1);
            if (str.size() > ret.size())
                ret = str;
                
            str = findPal(s, i, i + 1);
            if (str.size() > ret.size())
                ret = str;
        }
        
        return ret;
    }
};