Coding problem · Easy · Basics

Reverse an Integer

Easy Algorithms & data structures

Return n with its digits reversed, keeping the sign. Leading zeros disappear: 120 becomes 21. |n| < 10^9.

Example 1

Input: n = 123
Output: 321

Example 2

Input: n = -456
Output: -654

Example 3

Input: n = 120
Output: 21

Hints

Hint 1

Work with the absolute value and remember the sign.

Hint 2

Build the result digit by digit: result = result × 10 + n % 10.

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Solution and explanation

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Time O(digits) · Space O(1)

Peel digits off the end with % 10 and push them onto the result with × 10 +. Apply the original sign at the end.

Reference solution

function reverseInteger(n) {
  let x = Math.abs(n), result = 0;
  while (x > 0) { result = result * 10 + (x % 10); x = Math.floor(x / 10); }
  return n < 0 ? -result : result;
}

function reverseInteger(n: number): number {
  let x = Math.abs(n), result = 0;
  while (x > 0) { result = result * 10 + (x % 10); x = Math.floor(x / 10); }
  return n < 0 ? -result : result;
}

def reverse_integer(n: int) -> int:
    result = int(str(abs(n))[::-1])
    return -result if n < 0 else result

<?php

function reverseInteger(int $n): int
{
    $result = (int) strrev((string) abs($n));
    return $n < 0 ? -$result : $result;
}

class Solution {
    public int reverseInteger(int n) {
        int x = Math.abs(n), result = 0;
        while (x > 0) { result = result * 10 + x % 10; x /= 10; }
        return n < 0 ? -result : result;
    }
}

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