LeetCode 9. Palindrome Number

Description

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

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

Example 1:

Input: 121
Output: true

Example 2:

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

Follow up:

Coud you solve it without converting the integer to a string?

Explanation

Negative numbers are not palindrome numbers, e.g., -121. We can exclude negative numbers first.

Then by comparing the integer number and its reversed integer, we can determine whether the integer is a palindrome.

Java Solution

public class Solution {
  public boolean isPalindrome(int x) {
    if (x < 0 || x != 0 && x % 10 == 0) {
      return false;
    }

    // reverse process
    int reversed = 0;
    int original = x;
    while (x != 0) {
      reversed = reversed * 10 + x % 10;
      x = x / 10;
    }

    return reversed == original;
  }
}

Python Solution

class Solution:
    def isPalindrome(self, x: int) -> bool:
        
        if x < 0:
            return False

        reverse = 0
        original = x
        
        while x > 0:
            reverse = reverse * 10 + x % 10
            x = x // 10
            
        return reverse == original
  • Time complexity: O(N)
  • Space complexity: O(1)

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