Reverse integer

32-bit integer with its digits reversed.

Go

Time Complexity: $O(log_{10}(n))$ - each iteration divides the input by 10
Auxiliary space: $O(1)$ - constant amount of space

func reverse(x int) int {
    result, sign := 0, 1
 
    if (x < 0) {
        sign = -1
        x = -x
    }
    
    for x > 0 {
      remainder := x % 10;
      result = result * 10 + remainder
      x /= 10;
    }
    
    var int32Converted int = int(int32(result))
    
    if int32Converted != result {
      return 0
    }
    
    return result * sign
}

Typescript

Time complexity: $O(n)$ - n is a length of string representation of an input number
Auxiliary space: $O(n)$ - n is a length of string representation of an input number

function reverse(x: number): number {
  const reversed = x.toString().split("").reverse().join("");

  // 32-bit signed integer overflow results in 0
  if (parseInt(reversed) > 2 ** 31) {
    return 0;
  }

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

C++

Time Complexity: $O(log_{10}(n))$ - each iteration divides the input by 10
Auxiliary space: $O(1)$ - constant amount of space

class Solution {

public:
  int reverse(int n) {
    int reversed_number = 0, remainder;

    while (n != 0) {
      remainder = n % 10;
      
      if (reversed_number > INT_MAX/10 || reversed_number == INT_MAX/10 && remainder > 7) {
        return 0 ;
      }
    
      if (reversed_number < INT_MIN/10 || reversed_number == INT_MIN/10 && remainder < -8) {
        return 0 ;
      }
    
      reversed_number = reversed_number * 10 + remainder;
      n /= 10;
    }	  
    
    return reversed_number;
  }

};

Natural number solution:

Time Complexity: $O(1)$ - constant amount of space

class Solution {
public:
    uint32_t reverseBits(uint32_t n) {
    n = (n >> 1) & 0x55555555 | (n << 1) & 0xaaaaaaaa;
    n = (n >> 2) & 0x33333333 | (n << 2) & 0xcccccccc;
    n = (n >> 4) & 0x0f0f0f0f | (n << 4) & 0xf0f0f0f0;
    n = (n >> 8) & 0x00ff00ff | (n << 8) & 0xff00ff00;
    n = (n >> 16) & 0x0000ffff | (n << 16) & 0xffff0000;
    return n;
  }
}

thoughtbook

Reverse integer

Go

Typescript

C++

Natural number solution: