public class Solution {

     public String strStr(String haystack, String needle) {

         if(haystack == null)

             return null;

         if(needle.length() == 0)

             return haystack;

         int i,j = 0;

         for(i = 0;i < haystack.length() - needle.length() + 1;i++){

             for(j = 0;j < needle.length();j++){

                 if(haystack.charAt(i+j) != needle.charAt(j))

                     break;

             }

             if(j == needle.length())

                 return haystack.substring(i,haystack.length());

         }

         return null;

     }

 }