[Leetcode] 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.

 

Solution:

题意判断数字是否是回文串，只能用常数空间，不能转换成字符串，不能反转数字。

先计算数字的位数，分别通过除法和取余，提取出要对比的对应数字。

public class Solution {
    public boolean isPalindrome(int x) {
        if(x<0)
            return false;
        if(x==0)
            return true;
        int base=1;
        while(x/base>=10)
            base*=10;
        while(x>0){
            int first=x/base;             //left digit
            int last=x%10;                //right digit
            if(first!=last)
                return false;
            x=(x-first*base)/10;          //get number between first and last  
            base/=100;
        }
        return true;
    }
}