题目：

题解：

class Solution {
public:
    string shortestPalindrome(string s) {
        int n = s.size();
        vector<int> fail(n, -1);
        for (int i = 1; i < n; ++i) {
            int j = fail[i - 1];
            while (j != -1 && s[j + 1] != s[i]) {
                j = fail[j];
            }
            if (s[j + 1] == s[i]) {
                fail[i] = j + 1;
            }
        }
        int best = -1;
        for (int i = n - 1; i >= 0; --i) {
            while (best != -1 && s[best + 1] != s[i]) {
                best = fail[best];
            }
            if (s[best + 1] == s[i]) {
                ++best;
            }
        }
        string add = (best == n - 1 ? "" : s.substr(best + 1, n));
        reverse(add.begin(), add.end());
        return add + s;
    }
};