StrStr,判断一个字符串是不是另一个字符串的字串，并返回子串的位置

 public int strStr(String haystack, String needle) {

         if(haystack == null || needle == null)
         {
             return -1;
         }

         for (int i = 0; i <= (haystack.length()-needle.length()); i++)
         {
             int m = i;
             for (int j = 0; j < needle.length(); j++)
             {
                 if (needle.charAt(j) == haystack.charAt(m))
                 {
                     m++;
                     if((m-i) == needle.length())
                     {
                         return i;
                     }
                 }
                 else
                 {
                     break;
                 }
             }
         }
         return -1;
     }