leetcode5. Longest Palindromic Substring

leetcode5. Longest Palindromic Substring

题意：

给定一个字符串s，找到s中最长的回文子字符串。可以假设s的最大长度为1000。

思路：

O(n)遍历s，然后以每个char为回文的中心，向两边尽可能延长，以此找出最长的回文。
初始化left和right的时候，因为回文的特征直接跳过与该char相同的字符。

ac代码：

C++

class Solution {
public:
    string longestPalindrome(string s) {
        int len = s.length();
        int res = 0;
        int start = -1;
        for(int i = 0; i < len; i++)
        {
            int u = i;
            while(i<len-1 && s[i+1] == s[i]) i++;
            int l = u - 1;
            int r = i + 1;
            while(l>=0&&r<len&&s[l] == s[r]) {l--; r++;}
            //res = max(r - l - 1, res);
            if(r - l -1 > res)
            {
                res = r - l -1;
                start = l ;
            }
        }
        return s.substr(start + 1,res);
    }
};

python

class Solution(object):
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        start = -1
        ans = 0           
        i = 0
        while i < len(s):
            l = i - 1
            while i < len(s) - 1 and s[i+1] == s[i]:
                i += 1
            r = i + 1
            while l >= 0 and r < len(s) and s[l] == s[r]:
                l -= 1
                r += 1
            if r - l - 1 >= ans:
                ans = r - l - 1
                start = l
            i += 1
        return s[start + 1:start + 1 + ans]