【Leetcode】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.

方法一：先将数字反转，比较反转前后是否相等。但这种方法反转时可能发生溢出。

class Solution {
public:
    bool isPalindrome(int x) {
        if (x < 0) {
            return false;
        }
        return x == reverse(x);
    }
private:
    int reverse(int x) {
        unsigned ret = 0;
        while (x) {
            ret = ret * 10 + x % 10;
            x /= 10;
        }
        return ret;
    }    
};

方法二：逐位比较。

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