Implement strStr()

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