(leetcode) Implement strStr()

class Solution {
public:
    int strStr(string haystack, string needle) {
        int i,j;
		i = 0;
		j = 0;
        if(needle.empty()) return 0;

        if(haystack.length() < needle.length())
		{
			return -1;
		}
		int lenh,lenn;
		lenh = haystack.length();
		lenn = needle.length();

		while(i !=lenh && j!=lenn)
		{
			if(haystack[i] == needle[j])
			{
				++i;
				++j;
			}
			else
			{
				i = i - j +1;
				j = 0;
			}
		}

		return j == lenn ? i - j : -1;
    }
};