Implement strStr()

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

public class Solution {
    public String strStr(String haystack, String needle) {
        if(haystack==null) return null;
        int len1= haystack.length();
        int len2 = needle.length();
        if(needle==null||len2==0) return haystack;
        int i=0,j=0;
        char[] nee = needle.toCharArray();
        char[] hay = haystack.toCharArray();
        int[] next  = getNext(nee,len2);
        while(i<len1){
            if(j==-1||hay[i]==nee[j]){
                i++;
                j++;
                if(j==len2){
                    return haystack.substring(i-j);
                }
            }
            else{
                j= next[j];
            }
        }
        return null;
    }
    
    public int[] getNext(char[]nee,int len){
        int i=0,j=-1;
        int []next = new int[len];
        next[0]=-1;
        while(i<len-1){
            if(j==-1||nee[i]==nee[j]){
                i++;
                j++;
                next[i]=j;
            }
            else{
                 j=next[j];
            }
        }
        return next;
    }
}