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.

这道求最长回文子串的题目非常经典，也有非常多的解法。DP是其中非常经典的解法。具体解法有: 1:DP, 2:枚举中心,向两边走.

DP代码python代码会超时, C++代码如下:

class Solution {
public:
    string longestPalindrome(string s) {
        const int n = s.size();
        int maxlen = 1, start = 0;
        bool f[n][n];
        fill_n(&f[0][0],n*n,false);
        
    for (int j=0;j<n;j++){
        f[j][j] = true;
        for(int i=0;i<j;i++){
            f[i][j] = (s[i]==s[j]&&(i+1>j-1||f[i+1][j-1]));
            if (f[i][j] and j-i+1 > maxlen){
                maxlen = j-i+1;
                start = i;
            }
        }
    }
    return s.substr(start,maxlen);
    }
};

枚举中心:

class Solution(object):
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        if len(s)==0:
            return 0
        maxLen=1
        start=0
        for i in xrange(len(s)):
            if i-maxLen >=1 and s[i-maxLen-1:i+1]==s[i-maxLen-1:i+1][::-1]: #剪枝
                start=i-maxLen-1
                maxLen+=2
                continue

            if i-maxLen >=0 and s[i-maxLen:i+1]==s[i-maxLen:i+1][::-1]:
                start=i-maxLen
                maxLen+=1
        return s[start:start+maxLen]