leetcode--Implement strStr()

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

This is a simple problem, 

public class Solution {
    public String strStr(String haystack, String needle) {
        int hlen = haystack.length(), nlen = needle.length();
        String result = null;
        if(nlen <= hlen){
            if(nlen == 0)
                result = haystack;
            else{
                for(int i = 0; i < hlen - nlen + 1; ++i){
                    int j = 0;
                    for(; j < nlen; ++j){
                        if(haystack.charAt(i) == needle.charAt(j))
                            ++i;
                        else
                            break;
                    }
                
                    i = i - j;
                    //we found the first needle
                    if(j == nlen){
                        result = haystack.substring(i, hlen);
                        break;
                    }
                }
            }
        }
        return result;
    }
}