[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.

最长回文，对于每一个字符从中间向两边找，注意分奇数与偶数两种情况。总的时间复杂度为O(n^2)。另外Manacher算法复杂度为O(n)，Manacher算法好复杂的样子，智商捉急，得好好消化一下。先贴个简单的吧。

 1 class Solution {

 2 public:

 3     int getLPS(const string &s, int idx1, int idx2) {

 4         while (idx1 >= 0 && idx2 < s.length() && s[idx1] == s[idx2]) {

 5             --idx1;

 6             ++idx2;

 7         }

 8         return idx2 - idx1 - 1;

 9     }

10     

11     string longestPalindrome(string s) {

12         if (s.length() < 2) return s;

13         int lps = -1, lps1, lps2, pos;

14         for (int i = 0; i < s.length(); ++i) {

15             lps1 = getLPS(s, i, i);

16             lps2 = getLPS(s, i, i + 1);

17             if (lps1 > lps || lps2 > lps) {

18                 pos = i;

19                 lps = max(lps1, lps2);

20             }

21         }

22         if (lps & 0x1) return s.substr(pos-(lps-1)/2, lps);

23         else return s.substr(pos-(lps-2)/2, lps);

24     }

25 };