LeetCode in Erlang
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
-spec is_palindrome(X :: integer()) -> boolean().
is_palindrome(X) when X < 0 ->
false;
is_palindrome(X) ->
Rev = reverse_number(X, 0),
Rev == X.
-spec reverse_number(Num :: integer(), Rev :: integer()) -> integer().
reverse_number(0, Rev) ->
Rev;
reverse_number(Num, Rev) ->
Digit = Num rem 10,
NewRev = Rev * 10 + Digit,
reverse_number(Num div 10, NewRev).