【Leetcode】Longest Palindromic Substring

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.

以每个字符为中心，向两侧拓展，找最长回文子串。

class Solution {
public:
    string longestPalindrome(string s) {
        int n = s.size();
        int start = 0, step = 0, max_len = 0;
        for (int k = 0; k < n; ++k) {
            step = 0;
            while (k - step - 1 >= 0 && k + step + 1 < n && s[k - step - 1] == s[k + step + 1]) {
                ++step;
            }
            if (2 * step + 1> max_len) {
                max_len = 2 * step + 1;
                start = k - step;
            }
            step = 0;
            while (k + step + 1 < n && k - step >= 0 && s[k + step + 1] == s[k - step]) {
                ++step;
            }
            if (2 * step > max_len) {
                max_len = 2 * step;
                start = k - step + 1;
            }
        }
        return s.substr(start, max_len);
    }
};