LeetCode-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:

public class Solution {
    public boolean isStrobogrammatic(String num) {
        int p1 = 0, p2 = num.length() - 1;
        while (p1 <= p2) {
            if (!isValidPair(num.charAt(p1), num.charAt(p2)))
                return false;
            p1++;
            p2--;
        }
        return true;
    }

    public boolean isValidNum(int num) {
        if (num == 0 || num == 1 || num == 6 || num == 8 || num == 9)
            return true;

        return false;
    }

    public boolean isValidPair(char c1, char c2) {
        int num1 = c1 - '0';
        int num2 = c2 - '0';
        if (!isValidNum(num1) || !isValidNum(num2))
            return false;

        if ((num1 == 6 && num2 != 9) || (num1 == 9 && num2 != 6)) {
            return false;
        } else if (num1 + num2 != 15 && num1 != num2) {
            return false;
        }
        return true;
    }
}