Palindrome Number

Problem

https://leetcode.com/problems/palindrome-number/

Given an integer x, return true if x is a palindrome, and false otherwise.

Example 1:

Input: x = 121
Output: true
Explanation: 121 reads as 121 from left to right and from right to left.

Example 2:

Input: x = -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

Example 3:

Input: x = 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Constraints:

-2:sup:`31``` <= x <= 2``:sup:`31``` - 1``

Follow up: Could you solve it without converting the integer to a string?

Pattern

Math

Approaches

Digit Extraction

Explanation

Extract the digits from a number and store them in a list digits. Then compare the list with its reverse digits[::-1] to check if the number is a palindrome.

Code

def isPalindrome(x: int) -> bool:
    """Check if a number is a palindrome."""
    if x < 0:
        return False

    digits = []
    while x > 0:
        digit = x % 10
        x //= 10
        digits.append(digit)

    return digits == digits[::-1]

Test

>>> from palindrome_number__digit_extraction import isPalindrome
>>> isPalindrome(121)
True
>>> isPalindrome(-121)
False
>>> isPalindrome(10)
False

Complexity

Measure	Complexity	Notes
Time	\(O(\log_{10} x)\)	extract each digit and then compare the list with its reverse
Auxiliary Space	\(O(\log_{10} x)\)	store digits list

palindrome_number__digit_extraction.isPalindrome(x: int) → bool: Check if a number is a palindrome.