A Number After a Double Reversal

LeetCode 2238 | Difficulty: Easy

Easy

Problem Description

Reversing an integer means to reverse all its digits.

- For example, reversing `2021` gives `1202`. Reversing `12300` gives `321` as the **leading zeros are not retained**.

Given an integer num, reverse num to get reversed1, then reverse reversed1 to get reversed2. Return true if reversed2 equals num. Otherwise return false.

Example 1:

Input: num = 526
Output: true
Explanation: Reverse num to get 625, then reverse 625 to get 526, which equals num.

Example 2:

Input: num = 1800
Output: false
Explanation: Reverse num to get 81, then reverse 81 to get 18, which does not equal num.

Example 3:

Input: num = 0
Output: true
Explanation: Reverse num to get 0, then reverse 0 to get 0, which equals num.

Constraints:

- `0 <= num <= 10^6`

Topics: Math

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Approach

Mathematical

Look for mathematical patterns or formulas. Consider: modular arithmetic, GCD/LCM, prime factorization, combinatorics, or geometric properties.

When to use

Problems with clear mathematical structure, counting, number properties.

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Solutions

Solution 1: C# (Best: 36 ms)

Metric	Value
Runtime	36 ms
Memory	28.6 MB
Date	2021-12-27

Solution

public class Solution {
    public bool IsSameAfterReversals(int num) {
        string str = Convert.ToString(num);
        int lo = 0, hi = str.Length - 1;
        if(num == 0 ) return true;
        if(hi>= 0 && str[hi] == '0') return false;
        
        return true;
    }
}

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Complexity Analysis

Approach	Time	Space
Solution	$O(n)$	$O(1) to O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• LeetCode provides 1 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: Other than the number 0 itself, any number that ends with 0 would lose some digits permanently when reversed.