public static String add(String str1, String str2) {

          if(str1 == null)

              return str2;

          if(str2 == null)

              return str1;

          StringBuffer s1 = new StringBuffer(str1).reverse();

          StringBuffer s2 = new StringBuffer(str2).reverse();

          StringBuffer res = new StringBuffer();

          int len1 = s1.length();

          int len2 = s2.length();

          int len;

          if(len1 < len2) {

              len = len2;

              int count = len2 - len1;

              while(count-- > 0)

                  s1.append('0');

          } else {

              len = len1;

              int count = len1 - len2;

              while(count-- > 0)

                  s2.append('0');

          }

          int overflow = 0;

          int num;

          for(int i = 0; i < len; i++) {

              num = s1.charAt(i) - '0' + s2.charAt(i) - '0' + overflow;

              if(num >= 10) {

                  overflow = 1;

                  num -= 10;

              } else {

                  overflow = 0;

              }

              res.append(String.valueOf(num));

          }

          if(overflow == 1)

              res.append(1);

          return res.reverse().toString();

     }