Strobogrammatic Number

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Write a function to determine if a number is strobogrammatic. The number is represented as a string.

For example, the numbers “69”, “88”, and “818” are all strobogrammatic.

Solution:

  1. public class Solution {
  2.   public boolean isStrobogrammatic(String num) {
  3.     if (num == null)
  4.       return false;
  6.     return helper(num, 0, num.length() - 1);
  7.   }
  9.   boolean helper(String s, int lo, int hi) {
  10.     if (lo > hi)
  11.       return true;
  13.     char c1 = s.charAt(lo);
  14.     char c2 = s.charAt(hi);
  16.     int mul = (c1 - '0') * (c2 - '0');
  18.     if (mul == 1 || mul == 54 || mul == 64 || (mul == 0 && c1 == c2))
  19.       return helper(s, lo + 1, hi - 1);
  21.     return false;
  22.   }
  23. }