Reverse digits of an integer

Question

how to reverse a number?

Example1: x = 123, return 321 Example2: x = -123, return -321

this is my answer:

public int reverse(int x) {
    int result = 0;
    while(x != 0){
        result = result * 10 + x % 10;
        x = x / 10;
    }
    return result;
}

but when I input 1534236469 , it will output 1056389759 , this is wrong. what do you think about my program? thanks.

Answer 1

One reason your program cannot give the right answer is that you store result in an int but you expect to be able to reverse the number 1534236469. The correct answer would be 9646324351, but that number is greater than the largest possible value of an int so you end up with something else. Try long long or try using input with no more than 9 digits.

---

Followup: I suggested long long because that will fairly reliably give you an 8-byte integer. You may also get 8 bytes in a long, depending on where you are building your code, but Visual C++ on 32-bit Windows (for example) will give you only 4 bytes. Possibly the 4-byte long will go the way of the 2-byte int soon enough, but at this point in time some of us still have to deal with it.

Answer 2

Jason, You should just change the type from int to long.

   public long  reverse(long x)
    {
        long result = 0;
        while (x != 0)
        {
            result = result * 10 + x % 10;
            x = x / 10;
        }
        return result;
    }

Answer 3

You can write x >0 (doesn't matter though )also after that you have to consider negative numbers , I made that change to your logic as follows (Also use long long to avoid overflow):

      long long reverse(long long x)
      {

           int sign = 1;
           long long ans=0;
           if(x < 0)
             sign = -1;
           x = abs(x);
           while(x > 0)
           {
               ans *= 10;
               ans += x%10;
               x /=10;
           }
           return ans*sign;
      }

Answer 4

How about convert to string and reverse? Quite simple:

   int reverseDigits(int x) {
      String s = Integer.toString(x);
      for (int i = 0; i < s.length() / 2; i++) {
        char t = s[i];
        s[i] = s[s.length() - i - 1];
        s[s.length() - i - 1] = t;
      }
      return Integer.parseInteger(s); // subject to overflow
   }

Answer 5

can use long type to store the result

public int reverse(int x) {
    long result = 0;
    while (x != 0) {
        result = result * 10 + x % 10;
        x /= 10;
    }
    if (result > Integer.MAX_VALUE || result < Integer.MIN_VALUE)
        return 0;
    return (int)result;
}

Answer 6

This is a question posted on Leetcode and it gives a wrong answer expecting a 0. The clue is that before returning the reversed integer we have to check if it does not exceed the limit of a 32-bit int ie 2^31-1.

Code in Python 3:

class Solution:
def reverse(self, x: int) -> int:
    s=[]
    rev=0
    neg=False
    if x==0:
        return 0
    if x<0:
        x=x* -1
        neg=True
    while x:
        s.append(x%10)
        x=int(x/10)
    i=len(s)
    j=0
    while i:
        rev=rev+s[j]*10**(i-1)
        i=i-1
        j=j+1
    if(rev>2**31-1):
        return 0
    return rev * -1 if neg else rev

Answer 7

Why not simply do:

while (x)
  print x%10
  x /= 10

with a double sign conversion if the value of x is originally negative, to avoid the question of what mod a -ve number is.

Answer 8

You are using int for storing the number whereas number is out of range of int. You have tagged algorithm in this question. So, better way would be by using link list. You can google more about it. There are lot of algorithms for reversing a link list.

Answer 9

A shorter version of Schultz9999's answer:

int reverseDigits(int x) {
  String s = Integer.toString(x);
  s=new StringBuilder(s).reverse().toString();
  return Integer.parseInt(s); 
}

Answer 10

Here is the python code of reverse number::

n=int(input('Enter the number:'))
r=0

while (n!=0):        
    remainder=n%10
    r=remainder+(r*10)
    n=n//10       

print('Reverse order is %d'%r)

Answer 11

A compact Python solution is

reverse = int(str(number)[::-1])

If negative numbers are a possibility, then

num = abs(number)          # absolute value of the number
rev = int(str(num)[::-1])  # reverse the number
reverse = -rev             # negate the reverse

Answer 12

In JS I wrote it in this way

function reverseNumber(n) {
  const reversed = n
    .toString()
    .split('')
    .reverse()
    .join('');

  return parseInt(reversed) * Math.sign(n);
}

Answer 13

Reverse Integer In JavaScript | Accepted LeetCode solution | Memory efficient

If reversing the number causes the value to go outside the signed 32-bit integer range [-2^31, 2^31 - 1], then returned 0.

Intuition: First converted the integer to a string which is much easy to reverse and check characters.

Approach: Converted number to string. Checked for 1st character negative value. Spliced (-) and stored if any which is concat in the last. Then reversed the string without (-).

    var reverse = function(x) {
    x= x.toString();
    let s = Number(x[0]) ? '' : x[0],reverse='';
    if(s) { //If x= -123 && here s='-'
        x =x.substring(1) // removing '-' from the string
    }
    for(let i = x.length-1; i>=0; i--) {
        if((Number(x[i]) && !reverse) || reverse){
            reverse += x[i];
        }
    }
    if(Number(s+reverse) > 2147483648 || (Number(s+reverse) < -2147483648 && Number(s+reverse) < 0)){
        return 0
    }
    return Number(s+reverse); // s='-' or ''
};