[LeetCode] 9. Palindrome Number ☆

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.

解法: 

　　这道验证回文数字的题不能使用额外空间，意味着不能把整数变成字符，然后来验证回文字符串。而是直接对整数进行操作，我们可以利用取整和取余来获得我们想要的数字，比如 1221 这个数字，如果 计算 1221 / 1000， 则可得首位1， 如果 1221 % 10， 则可得到末尾1，进行比较，然后把中间的22取出继续比较。代码如下：

public class Solution {
    public boolean isPalindrome(int x) {
        if (x < 0) return false;
        
        int div = 1;
        while (x / div >= 10) div *= 10;  // 不能写成while(div*10<=x)，因为乘法可能溢出
        while (x > 0) {
            int left = x / div;
            int right = x % 10;
            if (left != right) return false;
            x = (x % div) / 10;
            div /= 100;
        }
        return true;
    }
}