Strobogrammatic Number

A strobogrammatic number is one that looks the same when rotated 180 degrees. Given a string num, return whether it is strobogrammatic.

Example 1

Input: "69" → Output: true

Example 2

Input: "962" → Output: false

Solution Approach

Check pairs from outside in. Valid strobogrammatic digit pairs: (0,0),(1,1),(6,9),(8,8),(9,6). Middle digit (if odd length) must be 0, 1, or 8.

Two pointers lo=0, hi=n-1.
Check if (num[lo], num[hi]) is a valid strobogrammatic pair.
Middle element (lo==hi) must be 0, 1, or 8.

Java

import java.util.*;

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 lo=0, hi=num.length()-1;
        while (lo <= hi) {
            char c = num.charAt(lo);
            if (!map.containsKey(c) || map.get(c) != num.charAt(hi)) return false;
            lo++; hi--;
        }
        return true;
    }
}

Time Complexity: O(N)
Space Complexity: O(1)