扩展kmp

void pre_ekmp(char s[], int ekmp[], int n)
{
    ekmp[0] = n; //国内是这样用的，oiwiki上好像是定义为0
    for (int i = 1, l = 0, r = 0; i < n; ++i)
    {
        if (i <= r) ekmp[i] = min(r - i + 1, ekmp[i - l]);
        while (i + ekmp[i] < n && s[ekmp[i]] == s[ekmp[i] + i]) ++ekmp[i];
        if (i + ekmp[i] - 1 > r) l = i, r = i + ekmp[i] - 1;
    }
}