LeetCode 009 Palindrome Number

题目描述：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.

分析：

题目要求without extra space，也就是说我们不能将整数转换成字符串，再进行比较。正确的思路为：不断取整数的最高位和最低位进行比较。

代码如下：

class Solution {
public:
    bool isPalindrome(int x) {
        
        if(x < 0 ) return false;
        if(x > 0 && x < 10) return true;
        
        //通过d找到最高位，先循环
        int d = 1;
        while(x / d >= 10) d *= 10;
        
        while(x != 0){
            
            //q为最高位，r为最低位
            int q = x / d;
            int r = x % 10;
            
            if(q != r) return false;
            
            x = x % d / 10;
            d = d / 100;
        }
        
        return true;
    }
};