class Solution {
public:
    int strStr(string haystack, string needle) {
        int n = haystack.size(),m = needle.size();
        if(!n && !m || !m) return 0;
        for(int i = 0;i < n;i ++)
        {
            if(haystack[i] == needle[0])
            {
                string s = haystack.substr(i,m);
                if(s == needle) return i;
            }
        }
        return -1;
    }
};