5. Longest Palindromic Substring

5. Longest Palindromic Substring

Medium

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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"

class Solution {
    int start;
    int len;
public:
    string longestPalindrome(string &s) {
        if(s.size()<2)return s;   //下面的循环处理不了空串和1个字符的串.
        start=s.size();
        len=0;
        for(int i=0;i<(int64_t)s.size()-1;++i)  //这里需要注意的是 size返回的是64位无符号, 如果size为0, 这里将变为非常大的正数,导致无限循环超时,所以用int64_t , 巨坑
        {
            validatePalindrome(s,i,i);
            validatePalindrome(s,i,i+1);
        }
        return s.substr(start,len);
    }
    
    void validatePalindrome(const string &s, int i, int j)
    {
        while(i>=0&&j<s.size()&&s[i]==s[j])
        {
            --i;
            ++j;
        }
        int l=j-i-1;
        if(l<=len)
            return;
        start=i+1;
        len=l;
    }
};