Reverse Integer

LeetCode 7 | Difficulty: Medium

Medium

Problem Description

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-2^31, 2^31 - 1], then return 0.

Assume the environment does not allow you to store 64-bit integers (signed or unsigned).

Example 1:

Input: x = 123
Output: 321

Example 2:

Input: x = -123
Output: -321

Example 3:

Input: x = 120
Output: 21

Constraints:

- `-2^31 <= x <= 2^31 - 1`

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: 63 ms)

Metric	Value
Runtime	63 ms
Memory	N/A
Date	2017-07-16

Solution

public class Solution {
    public int Reverse(int x) {
        bool isNeg = false;
    if (x < 0)
    {
        isNeg = true;
    }
    int rev = 0;
    while (x != 0)
    {
        try
        {
            int rem = x % 10;
            x /= 10;
            rev = checked(rev * 10 + rem);
        }
        catch (OverflowException)
        {
            return 0;
        }
        
    }
    
    return rev;
    }
}

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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

• Discuss the brute force approach first, then optimize. Explain your thought process.