[leetcode] 9 Palindrome Number

https://leetcode.com/problems/palindrome-number/

Determine whether an integer is a palindrome. Do this without extra space.

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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.

主要的问题

• 不使用额外空间
• 负数处理
• 如果转换成string，那么需要额外空间
• overflow的处理
• Reverse Integer中的100，转001的情况

大体思路：利用x%10每次取得个数位上的数字，然后构建回文数，最后判断是否相等。
技巧：1）异常判断中100的处理，2）利用回文的对称性，只需要判断一半的位数就可以了。

class Solution {
public:
    bool isPalindrome(int x) {
        if (x<0||(x!=0 && x%10==0)) return false;
        int sum=0;
        //这里考虑样例 121， 2211的不同情况
        while(x > sum){
            sum = sum*10 + x%10;
            x = x/10;
        }
        return (x==sum) || (x==sum/10);
    }
};