246. Strobogrammatic Number

Description

Given a string num which represents an integer, return true if num is a strobogrammatic number.

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Constraints

• 1 <= num.length <= 50

• num consists of only digits.

• num does not contain any leading zeros except for zero itself.

Approach

Links

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

Example 1

Input: num = "69"

Output: true

Example 2

Example 3

Example 4

Solutions

Solution 1

/**
 * Time complexity : O(N)
 * Space complexity : O(N)
 */

class Solution {

    public boolean isStrobogrammatic(String num) {
        
        // Note that using a String here and appending to it would be
        // poor programming practice.
        StringBuilder rotatedStringBuilder = new StringBuilder();
        
        // Remember that we want to loop backwards through the string
        for (int i = num.length() - 1; i >= 0; i--) {
            char c = num.charAt(i);
            if (c == '0' || c == '1' || c == '8') {
                rotatedStringBuilder.append(c);
            } else if (c == '6') {
                rotatedStringBuilder.append('9');
            } else if (c == '9') {
                rotatedStringBuilder.append('6');
            } else { // This must be an invalid digit.
                return false;
            }
        }
        
        String rotatedString = rotatedStringBuilder.toString();
        return num.equals(rotatedString);
    }
}

Solution 2

Follow up