LeetCode-in-All

7. Reverse Integer

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).

Example 1:

Input: x = 123

Output: 321

Example 2:

Input: x = -123

Output: -321

Example 3:

Input: x = 120

Output: 21

Example 4:

Input: x = 0

Output: 0

Constraints:

Solution

defmodule Solution do
  @spec reverse(x :: integer) :: integer
  def reverse(x) do
    rev = reverse_digits(x, 0)
    if rev > :math.pow(2, 31) - 1 or rev < -(:math.pow(2, 31)) do
      0
    else
      rev
    end
  end

  defp reverse_digits(0, acc), do: acc

  defp reverse_digits(x, acc) do
    new_acc = (acc * 10) + rem(x, 10)
    reverse_digits(div(x, 10), new_acc)
  end
end