Reverse Integer - Practice Coding Problems

Reverse Integer - Problem

Math Medium

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 [-2³¹, 2³¹ - 1], then return 0.

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

Input & Output

Example 1 — Basic Positive Number

$ Input: x = 123

› Output: 321

💡 Note: Reverse the digits: 123 becomes 321. The result fits in 32-bit range so return 321.

Example 2 — Negative Number

$ Input: x = -123

› Output: -321

💡 Note: Reverse the digits while preserving the negative sign: -123 becomes -321.

Example 3 — Overflow Case

$ Input: x = 1534236469

› Output: 0

💡 Note: Reversing gives 9646324351, which exceeds 2³¹-1 = 2147483647, so return 0.

Constraints

-2³¹ ≤ x ≤ 2³¹ - 1

Visualization

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Understanding the Visualization

Input

32-bit signed integer x

Process

Reverse digits while checking overflow

Output

Reversed integer or 0 if overflow

Key Takeaway

🎯 Key Insight: Check for overflow before multiplying by 10 to avoid using 64-bit integers

Asked in

🍎 Apple 15 a Amazon 12 B Bloomberg 8 M Microsoft 6

The key insight is to extract digits using modulo and check for overflow before multiplying. The mathematical approach is optimal: build the reversed number digit by digit while checking 32-bit bounds. Time: O(log n), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ String Conversion	O(log n)	O(log n)	Convert to string, reverse it, convert back to integer
Mathematical Approach	O(log n)	O(1)	Extract digits using modulo and build reverse number mathematically

String Conversion — Algorithm Steps

Convert integer to string
Handle negative sign separately
Reverse the digit string
Convert back to integer
Check if result overflows 32-bit range

Visualization

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Step-by-Step Walkthrough

Convert to String

Convert integer to string representation

Reverse String

Reverse the character order

Convert Back

Parse back to integer and check bounds

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>

int solution(int x) {
    if (x == 0) return 0;
    
    int negative = x < 0;
    long long absX = abs(x);
    
    // Convert to string
    char str[20];
    sprintf(str, "%lld", absX);
    
    // Reverse string
    int len = strlen(str);
    for (int i = 0; i < len / 2; i++) {
        char temp = str[i];
        str[i] = str[len - 1 - i];
        str[len - 1 - i] = temp;
    }
    
    // Convert back to long long
    long long result = atoll(str);
    
    if (negative) {
        result = -result;
    }
    
    // Check 32-bit integer bounds
    if (result < INT_MIN || result > INT_MAX) {
        return 0;
    }
    
    return (int)result;
}

int main() {
    int x;
    scanf("%d", &x);
    printf("%d\n", solution(x));
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(log n)

Converting to string and reversing takes O(log n) time where log n is the number of digits

⚡ Linearithmic

Space Complexity

O(log n)

String storage requires O(log n) space for the digits

⚡ Linearithmic Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

String Conversion — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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