public String strStr(String haystack, String needle) {
      if(haystack == null || needle == null) {
          return null;
      } else if(needle.length() == 0) {
          return haystack;
      }
      
      for(int haystackIdx = 0; haystackIdx <= haystack.length() - needle.length(); haystackIdx++) {
          for(int needleIdx = 0; needleIdx < needle.length(); needleIdx++) {
              if(haystack.charAt(haystackIdx + needleIdx) != needle.charAt(needleIdx)) {
                  break;
              }
              if(needleIdx == needle.length() - 1) {
                  return haystack.substring(haystackIdx);
              }
          }
      }
      
      return null;
  }