LeetCode in Racket
9. Palindrome Number
Easy
Given an integer x, return true if x is palindrome integer.
An integer is a palindrome when it reads the same backward as forward. For example, 121 is palindrome while 123 is not.
Example 1:
Input: x = 121
Output: true
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.
Example 4:
Input: x = -101
Output: false
Constraints:
-231 <= x <= 231 - 1
Follow up: Could you solve it without converting the integer to a string?
Solution
(define/contract (is-palindrome x)
(-> exact-integer? boolean?)
(if (< x 0)
#f
(let loop ((localX x) (rev 0))
(if (= localX 0)
(= rev x)
(loop (quotient localX 10) (+ (* rev 10) (remainder localX 10)))))))