LeetCode Problem 9:Palindrome Number回文數

描述：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.

注意問題要求O(1)空間復雜度，注意負數不是回文數。

思路1：將輸入整數轉換成倒序的一個整數，再比較轉換前后的兩個數是否相等，但是這樣需要額外的空間開銷

思路2：每次提取頭尾兩個數，判斷它們是否相等，判斷后去掉頭尾兩個數。

class Solution {
public:
    bool isPalindrome(int x) {
        
        //negative number
        if(x < 0)
            return false;
            
        int len = 1;
        while(x / len >= 10)
            len *= 10;
            
        while(x > 0)    {
            
            //get the head and tail number
            int left = x / len;
            int right = x % 10;
            
            if(left != right)
                return false;
            else    {
                //remove the head and tail number
                x = (x % len) / 10;
                len /= 100;
            }
        }
        
        return true;
    }
};