Palindrome Number Python Solution with Tests

Tested Python solution for LeetCode 9 with 16 pytest cases. Generate a practice environment with lcpy.

LeetCode 9, Easy. Topics: Math. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 16 parametrized pytest cases, and a playground notebook:

lcpy gen -n 9   # by problem number
lcpy gen -s palindrome_number   # by problem name

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Problem

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

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Examples

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

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.

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

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Constraints

• -2^31 <= x <= 2^31 - 1

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

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(log10(n)) - process half the digits
    # Space: O(1)
    def is_palindrome(self, x: int) -> bool:
        # Negative numbers and numbers ending in 0 (except 0 itself) are not palindromes
        if x < 0 or (x % 10 == 0 and x != 0):
            return False

        reversed_half = 0
        while x > reversed_half:
            reversed_half = reversed_half * 10 + x % 10
            x //= 10

        # Even length: x == reversed_half
        # Odd length: x == reversed_half // 10 (drop middle digit)
        return x == reversed_half or x == reversed_half // 10

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Complexity

Time	Space
O(log10(n)) - process half the digits	O(1)

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Tags

Grind, NeetCode All.