字符串hash模板

const int seed = 31;
ull h[maxn];
ull base[maxn];
ull string_hash(int l, int r)
{
    return h[r] - h[l-1]*base[r-l+1];
}
void init(){
    for(int i = 1; i <= len; ++i) base[i] = base[i-1]*seed;
    h[0] = 0;
    for(int i = 1; i <= len; ++i)
        h[i] = h[i-1]*seed + s[i] - 'a';
}