class Solution {

public:

    int strStr(string haystack, string needle) {

        if(needle.empty())return ;    //needle empty

        if(haystack.empty()) return -;    //haystack empty

        for(int i = , j = ; i+j < haystack.size();) {

            if(haystack[i+j] != needle[j])i++, j = ;     //no equal   needle index to 0, haystack index move to next.

            else j++;   //equal both move to next

            if(j == needle.size())return i;     //thr first time find substr.

        }

        return -;

    }

};