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';}