class Solution {
    public int strStr(String haystack, String needle) {
        if(needle.length()>haystack.length())
            return -1;
        if(needle.length()==0)
            return 0;
        char[] hs=haystack.toCharArray();
        char[] ne=needle.toCharArray();
        int[] next=new int[ne.length];
        next[0]=-1;int j=0,k=-1;
        while(j<needle.length()-1){
            if(k==-1||ne[k]==ne[j]){
                next[++j]=++k;
            }else{
                k=next[k];
            }
        }
        int i=0,head;
        while(i<=(haystack.length()-needle.length())){
             head=i;
             int m=0;
             for(m=0;m<needle.length();m++){
                 if(ne[m]!=hs[i]){
                     i=head+(m-next[m]);
                     break;
                 }
                 i++;
             }
            if(m==needle.length()){
                return head;
            }
        }
        return -1;
    }
}