leetcode[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.

Update (2014-11-02):
The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button  to reset your code definition.

class Solution {

public:

    int strStr(char *haystack, char *needle) {

    if(haystack==NULL||needle==NULL)return -1;

    int len1=strlen(haystack);

    int len2=strlen(needle);

    if(len1<len2)return -1;

    int i=0;

    for(;i<(len1-len2+1);i++)

    {

        int j=0;

        for(;j<len2;j++)

        {

            if(haystack[i+j]!=needle[j])

                break;

        }

        if(j==len2)return i;

    } 

    return -1;

    }

};