28. Implement strStr()

int strStr(string haystack, string needle) {
    if(needle.size()==0) return 0;
    if (needle.size()>haystack.size()) return -1;

    for(int i=0,j=0;i<haystack.size();++i) {
        if(haystack[i]==needle[j]) {
            ++j;
        } else {
            i=i-j;
            j=0;
        }
        if (j==needle.size()) {
            return i-j+1;
        }
    }
    return -1;
}