Strobogrammatic Number - Practice Coding Problems

Strobogrammatic Number - Problem

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

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

When rotated 180 degrees, the digits transform as follows:

0 → 0
1 → 1
6 → 9
8 → 8
9 → 6
All other digits (2, 3, 4, 5, 7) are invalid when rotated

Input & Output

Example 1 — Valid Strobogrammatic

$ Input: num = "69"

› Output: true

💡 Note: When rotated 180°: '6' becomes '9' and '9' becomes '6', giving us '96'. Reversed: '69' equals original.

Example 2 — Invalid Digit

$ Input: num = "88"

› Output: true

💡 Note: Both '8' digits remain '8' when rotated 180°, so '88' rotated is still '88'.

Example 3 — Contains Invalid Digit

$ Input: num = "962"

› Output: false

💡 Note: The digit '2' cannot be rotated 180° to form a valid digit, so this is not strobogrammatic.

Constraints

1 ≤ num.length ≤ 50
num consists of only digits.
num does not contain any leading zeros except for the zero itself.

Visualization

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Understanding the Visualization

Input

Original number as string

Rotation

Each digit rotated 180° and string reversed

Comparison

Check if rotated equals original

Key Takeaway

🎯 Key Insight: Use two pointers to validate rotation pairs from both ends simultaneously

Asked in

G Google 45 f Facebook 38 M Microsoft 25

The key insight is that only digits 0,1,6,8,9 are valid when rotated 180°, and we can use two pointers to validate pairs efficiently. Best approach uses two pointers for O(1) space. Time: O(n), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Two Pointers Validation	O(n)	O(1)	Check rotation pairs from both ends moving inward
Build Rotated String	O(n)	O(n)	Create the rotated string and compare with original

Two Pointers Validation — Algorithm Steps

Place left pointer at start, right pointer at end
Check if current pair forms valid rotation
Move pointers toward center until they meet

Visualization

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Step-by-Step Walkthrough

Initialize

Set left pointer at start, right at end

Validate Pair

Check if current pair forms valid rotation

Move Inward

Move pointers toward center and repeat

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <stdbool.h>

bool solution(char* num) {
    int left = 0, right = strlen(num) - 1;
    
    while (left <= right) {
        char leftChar = num[left], rightChar = num[right];
        
        // Check if it's a valid strobogrammatic pair
        if ((leftChar == '0' && rightChar == '0') ||
            (leftChar == '1' && rightChar == '1') ||
            (leftChar == '6' && rightChar == '9') ||
            (leftChar == '8' && rightChar == '8') ||
            (leftChar == '9' && rightChar == '6')) {
            left++;
            right--;
        } else {
            return false;
        }
    }
    
    return true;
}

int main() {
    char num[1000];
    fgets(num, sizeof(num), stdin);
    num[strcspn(num, "\n")] = 0;
    
    bool result = solution(num);
    printf(result ? "true\n" : "false\n");
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass with two pointers meeting in the middle

✓ Linear Growth

Space Complexity

O(1)

Only using constant extra space for pointers and mapping

✓ Linear Space

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Smart Actions

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Two Pointers Validation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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