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.