[LeetCode]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.

[思路]典型的字符串查找。有很多有效方法比如:KMP有限状态机，字符指纹，Boyer-Moore算法。这些算法实现近似在O(N)。更简单的暴力匹配法在O(MN)解决，其他算法实现太过复杂，直接采用暴力匹配法解决。注意边界条件。

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