246 Strobogrammatic Number

246. Strobogrammatic Number

1. Question

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.

2. Implementation

(1) HashMap + Two Pointers

class Solution {
    public boolean isStrobogrammatic(String num) {
        Map<Character, Character> map = new HashMap<>();

        map.put('0', '0');
        map.put('1', '1');
        map.put('6', '9');
        map.put('8', '8');
        map.put('9', '6');

        int start = 0, end = num.length() - 1;
        while (start <= end) {
            if (!map.containsKey(num.charAt(start)) || map.get(num.charAt(start)) != num.charAt(end)) {
                return false;
            }
            ++start;
            --end;
        }
        return true;
    }
}

3. Time & Space Complexity

HashMap + Two Pointers: 时间复杂度O(n), 空间复杂度O(n)