LeetCode 5. Longest Palindromic Substring

题目链接 最长回文子串 - 力扣 (LeetCode)

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"

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题目范围小于1000所以可以用n^2的算法
枚举中心点，用两个指针向两个方向遍历
要分为回文字串长度是奇数和偶数的情况

class Solution {
public:
    string longestPalindrome(string s) {
        int len = -1;
        string ans;
        for(int pos = 0; pos < s.size(); ++ pos){
            // 回文字串长度为偶数
            int i = pos, j = pos + 1;
            while(~i && j < s.size() && s[i] == s[j]) i --, j ++;
            if(j - i - 1 > len){
                len = j - i - 1;
                ans = s.substr(i + 1, len);
            }
            
            i = pos - 1, j = pos + 1;
            while(~i && j < s.size() && s[j] == s[i]) i --, j ++;
            if(j - i - 1 > len){
                len = j - i - 1;
                ans = s.substr(i + 1, len);
            }
        }
        return ans;
    }
};