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1.将输入reverse下，看结果与输入是否相等，但是Reverse后的数可能溢出

public class Solution {
    public boolean isPalindrome(int x) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(x < 0) return false;
        
        int reverseX = reverseNum(x);
        
        if(x == reverseX)
            return true;
        else
            return false;
        
    }
    
    public int reverseNum(int x){
        boolean positive = (x > 0) ? true : false;
        int result = 0;
        x = Math.abs(x);
        while(x > 0){
            result = result * 10 + x % 10;
            x = x / 10;
        }
        if(!positive){
            result *= -1;
        }
        return result;
    }
}

 方法2.不断比较最高位与最低位，如果相等去除最高位和最低位，直到x = 0

public class Solution {
    public boolean isPalindrome(int x) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(x < 0) return false;
        
        int tmp = x;
        int s = 1;
        boolean flag = true;
        while(tmp / 10 > 0){
            s *= 10;
            tmp = tmp / 10;
        }
        
        while(x > 0){
            int quotient = x / s;
            int remainder = x % 10;
            if(quotient != remainder){
                flag = false;
                break;
            }
            
            x = x - quotient * s;
            x = x - remainder;
            x = x / 10;
            s = s / 100;
        }
        return flag;
    }
    
}

 方法2中代码23-25行重构如下

x = x % s /10;