leetcode 28 -- Implement strStr()

Implement strStr()

题目：
mplement 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.

题意：
找到第二个字符串在第一个字符串中出现的下标，返回即可。

思路：
轮寻第一个字符串s1，比较s1的字符和s2的第一个字符，如果相等，则strncmp(s1, s2, len(s2)) == 0 ?。不相等则s1++。

class Solution {
public:
    int strStr(string haystack, string needle) {
        int len = needle.length();
        if(len == 0){
            return 0;
        }
        if(haystack.length() == 0){
            return -1;
        }
        for(int i = 0; i < haystack.length(); ++i){
            if(haystack[i] == needle[0] && haystack.substr(i, len) == needle){
                return i;
            }
        }
        return -1;
    }
};