Leetcode 0246: Strobogrammatic Number

題目描述：

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.

Example 1:

Input: num = “69”

Output: true

Example 2:

Input: num = “88”

Output: true

Example 3:

Input: num = “962”

Output: false

Example 4:

Input: num = “1”

Output: true

Time complexity: O(n)

Space complexity : O(1)

class Solution {
    public boolean isStrobogrammatic(String num) {
        Map<Character, Character> map = new HashMap<> (
            Map.of('0', '0', '1', '1', '6', '9', '8', '8', '9', '6'));
        for(int l = 0, r = num.length()-1; l <= r; l++,r--){
            char lc = num.charAt(l);
            char rc = num.charAt(r);
            if(!map.containsKey(lc) || map.get(lc) != rc){
                return false;
            }
        }
        return true;
    }
}