# 28. Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

class Solution {
public:
    int strStr(string haystack, string needle) {
        int len_1 = haystack.size(),len_2 = needle.size(),j,num=0;
        if(needle == "")
            return 0;
        if(len_1<len_2)
            return -1;
        for(int i=0;i<len_1-len_2+1;i++){
            if(haystack[i]==needle[0]){
                num=0;
                for(j=0;j<len_2;j++){
                    if(haystack[i+j]!=needle[j]){
                        break;
                    }else{
                        num++;
                    }
                }
                if(num==len_2)
                    return i;
            }
        }
        return -1;
    }
};

不是很理解为什么要处理strStr(“”,”“)=0,strStr(“a”,”“)=0的情况，空字符串怎么会有匹配呢？