回文子串

O(n)

class Solution {
public:
    string longestPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        string s2 = "";
        for (int i = 0; i < s.size(); ++i)
            s2 = s2 + "#" + s[i];
        s2 += "#";
        int* r = new int[s2.size() + 2];
        memset(r, 0, s2.size() + 2);
        int maxPosition = 0, position = 0;
        int lp = 0, lr = 0;
        for (int i = 1; i < s2.size(); ++i) {
            if (i < maxPosition) {
                r[i] = maxPosition - i;
                if (r[i] > r[2 * position - i])
                    r[i] = r[2 * position - i];
            }
            else
                r[i] = 1;
            while ((i - r[i] > -1 && i + r[i] < 2 * s.size() + 1) && s2[i - r[i]] == s2[i + r[i]]) r[i]++;
            if (r[i] + i > maxPosition) {
                maxPosition = r[i] + i;
                position = i;
            }
            if (r[i] > lr) {
                lr = r[i];
                lp = i;
            }
        }
        string result = "";
        for (int i = lp - lr + 1; i < lp + lr; ++i) {
            if (s2[i] != '#')
                result += s2[i];
        }
        return result;
    }
};