LeetCode #9 Palindrome Number C# Solution

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

题目只有很简单的一句话，然而提示又是一大堆

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.

我只能说exciting。
负数都是false，而且不能使用额外空间，也即是说不能把这个数转成一个字符串或者数组来从两端判断，所以这个题就是拆出最低位和最高位。
设这个数的位数是n，那么我们设一个变量等于10^n，这样每次就可以整数除法取得最高位。最低位MOD10。要注意的是，因为每次循环都会减少最高位和最低位两位数所以我们需要每次对这个变量除以100而不是除以10。

    public class Solution
    {
        public bool IsPalindrome(int x)
        {
            if (x < 0)
                return false;
            if (x == 0)
                return true;

            int exciting = 1;
            while (x / exciting >= 10)
                exciting *= 10;

            while (x != 0)
            {
                if (x % 10 != (int)x / exciting) return false;
                x -= exciting * (int)(x / exciting);
                x /= 10;
                exciting /= 100;
            }
            return true;
        }
    }