How do you approach Palindrome Number?

Negative numbers cannot be palindromes.

What is the time complexity of Palindrome Number?

Time complexity: O(log n) - we process half the digits. Space complexity: O(1).

What are common mistakes on Palindrome Number?

Overflowing at an intermediate step even though the final answer fits. Assuming a modulo is non-negative. In most languages -1 % n is negative, which breaks cyclic indexing. Testing floating-point values for exact equality where integer arithmetic would have been exact.

gitGood.dev

← Back to problems

Palindrome Number

EasyAlgorithms

Description

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

An integer is a palindrome when it reads the same backward as forward.

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

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

- -2^31 <= x <= 2^31 - 1

Palindrome Number: approach, complexity and pitfalls

An easy math problem, graded against 7 test cases (4 of them hidden).

The pattern: Math

Number theory, digit manipulation, and closed-form reasoning.

Reach for it when you see: Primes, digits, base conversion, or a problem with a formula hiding behind it.

If you are stuck

Show the 3-step hint ladder

Negative numbers cannot be palindromes.
Try reversing only half of the number to avoid overflow.
How do you know when you've reversed half the number?

The optimal approach

Show the approach: Reverse Half

Reverse only the second half of the number and compare with the first half. This avoids string conversion and potential overflow.

Time: O(log n) - we process half the digits
Space: O(1)

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Negative numbers. The suite includes negative values. Running-sum, min/max seeding and any "initialise to zero" step need to handle them.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Math mistakes

These apply to the pattern as a whole, not just this problem.

Overflowing at an intermediate step even though the final answer fits.
Assuming a modulo is non-negative. In most languages `-1 % n` is negative, which breaks cyclic indexing.
Testing floating-point values for exact equality where integer arithmetic would have been exact.

← Back to problems

Palindrome Number

EasyAlgorithms

Description

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

An integer is a palindrome when it reads the same backward as forward.

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

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

- -2^31 <= x <= 2^31 - 1

Palindrome Number: approach, complexity and pitfalls

An easy math problem, graded against 7 test cases (4 of them hidden).

The pattern: Math

Number theory, digit manipulation, and closed-form reasoning.

Reach for it when you see: Primes, digits, base conversion, or a problem with a formula hiding behind it.

If you are stuck

Show the 3-step hint ladder

Negative numbers cannot be palindromes.
Try reversing only half of the number to avoid overflow.
How do you know when you've reversed half the number?

The optimal approach

Show the approach: Reverse Half

Reverse only the second half of the number and compare with the first half. This avoids string conversion and potential overflow.

Time: O(log n) - we process half the digits
Space: O(1)

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Negative numbers. The suite includes negative values. Running-sum, min/max seeding and any "initialise to zero" step need to handle them.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Math mistakes

These apply to the pattern as a whole, not just this problem.

Overflowing at an intermediate step even though the final answer fits.
Assuming a modulo is non-negative. In most languages `-1 % n` is negative, which breaks cyclic indexing.
Testing floating-point values for exact equality where integer arithmetic would have been exact.