leetcode--Palindrome Number

1.题目描述

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.

2.解法分析

不能用额外的空间就只能在回文的特点上做文章了，回文的特点是对称，那么如果能够把首尾成对的数字取出就好办了，这个算法很简单，尾部的数字可以不断对10取余，首部数字只要知道当前数字的位数就可以轻松取出，因此有下面的代码，需要注意的是，下面代码中来自cmath库的pow函数返回值是float，需要强制类型转换一下，否则得出的结果不对。

class Solution {
public:
    bool isPalindrome(int x) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(x<0)return false;
        if(!x)return true;
        int len=0;
        int xtemp=x;
        while(xtemp)
        {
            len++;
            xtemp/=10;
        }

        xtemp=x;

        for(int i=0;i<=(len+1)/2-1;i++)
        {
            if(xtemp/(int)pow(10,len-1-2*i)!=xtemp%10)return false;
            xtemp-=(int)pow(10,len-1-2*i)*(xtemp/(int)pow(10,len-1-2*i));
            xtemp/=10;
        }

        return true;

    }
};

当然，其他方法如果能取出首尾对称的数字也是可以的。