LeetCode Problem 9:Palindrome Number迴文數

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)空間複雜度，注意負數不是迴文數。

思路1：將輸入整數轉換成倒序的一個整數，再比較轉換先後的兩個數是否相等，可是這樣須要額外的空間開銷

思路2：每次提取頭尾兩個數，判斷它們是否相等，判斷後去掉頭尾兩個數。

 1 class Solution {
 2 public:
 3     bool isPalindrome(int x) {
 4         
 5         //negative number
 6         if(x < 0)
 7             return false;
 8             
 9         int len = 1;
10         while(x / len >= 10)
11             len *= 10;
12             
13         while(x > 0)    {
14             
15             //get the head and tail number
16             int left = x / len;
17             int right = x % 10;
18             
19             if(left != right)
20                 return false;
21             else    {
22                 //remove the head and tail number
23                 x = (x % len) / 10;
24                 len /= 100;
25             }
26         }
27         
28         return true;
29     }
30 };