LeetCode--5

5. Longest Palindromic Substring

题目描述

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"

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解题思路

1. 判断是否为空字符；
2. 判断是否为单字符；
3. 遍历字符，如果字符剩余长度小于已找到的最长回文长度的一半则终止，依次判断是否连续两个字符相等、判断是否叠字符相等；更新最长回文自字符。
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代码实现

class Solution {
public:
    string longestPalindrome(string s) {
        if(s.empty()) return "";//空字符
        else if(s.size()==1) return s;//单字符
        else{//长字符
            int start=0; 
            int maxlen=1;
            for(int i=0; i<s.size(); ){//遍历字符
                if(s.size()-i<=maxlen/2) break;//如果字符剩余长度小于已找到的最长回文长度的一半则终止
                int p=i;
                int k=i;
                while(k<s.size()-1 && s[k]==s[k+1]) ++k;//连续两个字符相同
                i=k+1;
                while(k<s.size()-1 && p>0 && s[k+1]==s[p-1]){//叠字符相同
                    ++k;
                    --p;
                }
                int newlen=k-p+1;
                if(newlen>maxlen){//判断是否大于已找到的字符长度
                    start=p;
                    maxlen=newlen;
                }
            }
            return s.substr(start,maxlen);
        }
    }
};