Question

link

Determine whether an integer is a palindrome. Do this without extra space.

Some hints:

Could negative integers be palindromes? (ie, -1)

If you are thinking of converting the integer to string, note the restriction of using extra space.

You could also try reversing an integer. However, if you have solved the problem "Reverse Integer", you know that the reversed integer might overflow. How would you handle such case?

There is a more generic way of solving this problem.

Stats

Frequency 2
Diffficulty 2
Adjusted Difficulty 2
Time to use ----------

Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)

Analysis

This question is easy, and very similar to the “Reverse Integer” question. Simpliest solution is to just reverse the integer and compare with original number.

But again, reversing the integer have chances of overflow. How do we work this out?

Solution

First, same as previous post [LeetCode 7] Reverse Integer, we can use Long type to handle the overflow issues.

Second, we can also do direct compare to always compare the head and tail of the number. No numerical type conversion is needed. I came up with idea previously, and code is posted below.

My code

Use Long type:

public class Solution {
    public boolean isPalindrome(int x) {
        if (x < 0) {
            return false;
        }
        long rev = 0;
        long original = x;
        while (x != 0) {
            rev = rev * 10 + (x % 10);
            x = x / 10;
        }
        return original == rev;
    }
}

Compare head and tail:

public boolean isPalindrome(int x) {
    if (x < 0) {
        return false;
    }
    int divide = 1;
    while (x / divide >= 10) {
        divide *= 10;
    }
    int head = 0, tail = 0;
    while (divide > 0) {
        head = x / divide;
        tail = x % 10;
        if (head != tail) {
            return false;
        }
        x = x % divide / 10;
        divide /= 100;
    }
    return true;
}