28. Implement strStr()

class Solution {
public:
    const unsigned long long T = 100000007;
    int strStr(string haystack, string needle) {
        typedef unsigned long long ull;
        int al = needle.length();
        int bl = haystack.length();
        if (al > bl) return -1;
        ull A = 0;
        ull B = 0;
        ull t = 1;
        for (int i = 0; i < al; ++i) {
            A = A * T + needle[i];
            B = B * T + haystack[i];
            t *= T;
        }
        //cout << A << " "<<B<<endl;
        for (int i = 0; i + al <= bl; ++i) {
            if (A == B) return i;
            if (i + al < bl) B = B * T - t * haystack[i] + haystack[i + al];
        }
        return -1;
    }
};