题目：

题解：

char* shortestPalindrome(char* s) {
    int n = strlen(s);
    int fail[n + 1];
    memset(fail, -1, sizeof(fail));
    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;
        }
    }
    int ret_len = n - best - 1;
    char* ret = malloc(sizeof(char) * (ret_len + n + 1));
    for (int i = 0; i < ret_len; i++) ret[i] = s[n - i - 1];
    for (int i = 0; i < n; i++) ret[i + ret_len] = s[i];
    ret[ret_len + n] = 0;
    return ret;
}