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         int p1 = 0, p2 = num.length() - 1; 4         while (p1 <= p2) { 5             if (!isValidPair(num.charAt(p1), num.charAt(p2))) 6                 return false; 7             p1++; 8             p2--; 9         }10         return true;11     }12 13     public boolean isValidNum(int num) {14         if (num == 0 || num == 1 || num == 6 || num == 8 || num == 9)15             return true;16 17         return false;18     }19 20     public boolean isValidPair(char c1, char c2) {21         int num1 = c1 - '0';22         int num2 = c2 - '0';23         if (!isValidNum(num1) || !isValidNum(num2))24             return false;25 26         if ((num1 == 6 && num2 != 9) || (num1 == 9 && num2 != 6)) {27             return false;28         } else if (num1 + num2 != 15 && num1 != num2) {29             return false;30         }31         return true;32     }33 }