class Solution {public:    int strStr(string haystack, string needle) {        if(needle.empty())return 0;    //needle empty         if(haystack.empty()) return -1;    //haystack empty        for(int i = 0, j = 0; i+j < haystack.size();) {            if(haystack[i+j] != needle[j])i++, j = 0;     //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 -1;    }};