Description

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

分析

题目的意思是：找出一个字符串中的最长回文子串。

• 这道题从中间开花，遍历每个结点，把每个节点都当作回文子串的中心位置，然后向左右拓展，找出所有这种回文子串中最长的就够了。

代码

class Solution {
public:
    string longestPalindrome(string s) {
        int max_len=0;
        int start=0;
        for(int i=0;i<s.length();i++){
            int left=i-1;
            int right=i+1;
            while(left>=0&&right<s.length()&&s[left]==s[right]){
                int cur_len=right-left;
                if(cur_len>max_len){
                    max_len=cur_len;
                    start=left;
                }
                left--;
                right++;
            }
            
            left=i;
            right=i+1;
            while(left>=0&&right<=s.length()&&s[left]==s[right]){
                int cur_len=right-left;
                if(cur_len>max_len){
                    max_len=cur_len;
                    start=left;
                }
                left--;
                right++;   
            }
        }
        return s.substr(start,max_len+1);
    }
};

参考文献

​​Manacher’s Algorithm 马拉车算法​​​​LeetCode 5. Longest Palindromic Substring（最长回文连续子串）​​​​[Leetcode] Longest palindromic substring 最长回文子串​​​​Longest Palindromic Substring Part II​​