5. Longest Palindromic Substring

问题描述：

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"

解题思路：

从文中可以看出，题目让求得是最长回文字符串，那么，我们将利用从两边扩散的思想，最大值为2*i+1，遇奇数比较两边，遇偶数，比较前一个与自己。

class Solution {
public:
    string longestPalindrome(string s) {
        int Max=1,Maxf=0,Maxe=0;
        for(int i=0;i<s.size();i++)
        {
            int end=findOdd(s,i);
            if(Max<(end-i)*2+1){
                Max=(end-i)*2+1;
                Maxf=i+i-end;
                Maxe=end;
            }
            end=findEven(s,i);
            if(Max<(end-i)*2){
                Max=(end-i)*2;
                Maxf=i+i+1-end;
                Maxe=end;
            }
        }
        return s.substr(Maxf,Max);
    }
    int findOdd(string s,int center){
        int i=center-1,j=center+1;
        while(i>=0&&j<s.size()){
            if(s[i]!=s[j]) return j-1;
            i--;j++;
        }
        return j-1;
    }
    int findEven(string s,int center){
        int i=center,j=center+1;
        while(i>=0&&j<s.size()){
            if(s[i]!=s[j])return j-1;
            i--;j++;
        }
        return j-1;
    }
};