Palindrome Number

题目：
Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:

Input: 121
Output: true

Example 2:

Input: -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

Example 3:

Input: 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Follow up:

Coud you solve it without converting the integer to a string?
注意：判断是否是回文。
方法一：会超时

 class Solution {
    public:
        bool isPalindrome(int x) {
            if(x<0) return false;
            int y,m=0;
            int mm;
            mm=x;
            while(mm!=0)
            {
                y=mm%10;
                mm=mm/10;
                m=m*10+y;
            }
            if(m==x)
                return true;
            else 
                return false;
        }
    };

方法二：利用整和取余来判断，头取整，尾取余。整数取最后一位用整数本身和10取余，取第一位则用整数除以位数相同的10的n次方。

  class Solution {
public:
    bool isPalindrome(int x) {
        if(x<0) return false;
        int m=x;
        int div=1;
        while(m/div >= 10) div=div*10;
        while(m>0)
        {
            int left=m/div;
            int right=m%10;
            if(left == right) 
            {
                m=(m%div)/10;
                div=div/100;
            }
            else return false; 
        }
        return true;
}
};

方法三：折半的方法。首先将特殊情况给定，然后看前后半段是否相同。

class Solution {
public:
    bool isPalindrome(int x) {
        if(x<0 || x%10 == 0 && x!=0) return false;
    int y=0;
        while(x>y)
        {
            y=y*10+x%10;
            x=x/10;
        }
        return x==y || x==y/10;
    }
};