246. 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:

There are only a few characters that are valid:

6 -> 9
9 -> 6
0 -> 0 or itself
8 -> 8 or itself
1 -> 1 or itself

We can use a HashMap to record this mapping, and use two pointers to check from two sides of the string to the middle.



Code:

public class Solution {
    public boolean isStrobogrammatic(String num) {
        HashMap<Character, Character> map = new HashMap<>();
        map.put('6', '9');
        map.put('9', '6');
        map.put('8', '8');
        map.put('1', '1');
        map.put('0', '0');
        int i = 0;
        int j = num.length() - 1;
        while (i <= j) {
            if (!map.containsKey(num.charAt(i))) {
                return false;
            }
            if (map.get(num.charAt(i)) != num.charAt(j)) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
}