LeetCode -- Longest Palindromic Substring

链接：http://leetcode.com/onlinejudge#question_5

原题：

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：之前，我都是用O（n^2）做的，这回尝试一下用O（n）时间做一下，当然，方法是别人想出来的，呵呵。

请参考：http://www.felix021.com/blog/read.php?2040

真是算法无止境啊，这碗水很深。

代码：

class Solution {
public:
    string longestPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (s.size() == 0)
            return "";
        
        string str = "#";
        for (int i=0; i<s.size(); i++) {
            str.append(1, s[i]);
            str.append(1, '#');
        }
        
        vector<int> state(str.size());
        state[0] = 1;
        int center = 0;
        int rBorder = center + state[0];
        
        for (int i=1; i<str.size(); i++) {
            int j = 2 * center - i;
            if (j < 0) {
                state[i] = 1;
                int n = 1;
                while (i-n >= 0 && i+n < str.size() && str[i-n] == str[i+n])
                    n++;
                state[i] += n - 1;
            } else {
                if (rBorder - i > state[j])
                    state[i] = state[j];
                else {
                    state[i] = rBorder - i;
                    int n = rBorder - i;
                    while (i-n >= 0 && i+n < str.size() && str[i-n] == str[i+n]) {
                        n++;
                        state[i]++;
                    }
                }
            }
            
            if (i + state[i] > rBorder) {
                rBorder = i + state[i];
                center = i;
            }
        }
        
        int maxLength = 1;
        int maxCenter = 0;
        for (int i=0; i<state.size(); i++) {
            if (state[i] > maxLength) {
                maxLength = state[i];
                maxCenter = i;
            }
        }
        
        maxLength;
        string palindrome = "";
        for (int i=maxCenter-maxLength+1; i<maxCenter+maxLength; i++) {
            if (str[i] != '#')
                palindrome += str[i];
        }
        
        return palindrome;
    }
};