LeetCode 005: Longest Palindromic Substring

005. Longest Palindromic Substring

Difficulty: Medium
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.

思路

回文可分为两种情况：AA’和AbA’（A为字符串，A’为A的反转字符串，b为一个字符）。
对于字符串s的每个字符，分别计算把它当作A的末尾字符和b时可取得的最长回文。
使用string的substr方法获取该回文子串。

代码

[C++]

class Solution {
public:
    string longestPalindrome(string s) {
        if(s.length() <= 1) return s;
        string res;
        for (int i = 0; i < s.length(); ++i) {
            string s1, s2;
            s1 = longestPalindromeCore(s, i, i + 1);
            s2 = longestPalindromeCore(s, i , i);
            if (res.length() < s1.length()) 
                res = s1;
            if (res.length() < s2.length()) 
                res = s2;
            if(res.length() == s.length())
                break;
        }
        return res;
    }
    string longestPalindromeCore(string &s, int begin, int end) {
        while (begin >= 0 && end < s.length()) {
            if(s[begin] != s[end]) 
                break;
            begin--;
            end++;
        }
        return s.substr(begin + 1, end - 1 - begin);
    }
};