5. Longest Palindromic Substring

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本文介绍了一种求解给定字符串中最长回文子串的方法——左右扩展法。该算法通过遍历字符串，针对每个字符为中心进行左右扩展判断是否构成回文串，并更新最长回文子串。适用于字符串最大长度为1000的情况。

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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, and there exists one unique longest palindromic substring.

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求解最长回文字串。左右扩展法

class Solution {
public:
    string longestPalindrome(string s) {
        if (s.size() < 2)
            return s;
        int len = s.size(), max_left = 0, max_len = 1, left, right;
        for (int start = 0; start < len && len - start > max_len / 2;) {
            left = right = start;
            while (right < len - 1 && s[right + 1] == s[right])
                ++right;
            start = right + 1;
            while (right < len - 1 && left > 0 && s[right + 1] == s[left - 1]) {
                ++right;
                --left;
            }
            if (max_len < right - left + 1) {
                max_left = left;
                max_len = right - left + 1;
            }
        }
    return s.substr(max_left, max_len);
    }
};