[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.

思路1（不推荐）：利用Reverse Integer的方法，求的转换后的数字，而后比较是否相等。提示说这样有溢出的问题，想了想感受问题不大，leetcode也过了。由于首先输入必须是一个合法的int值，负数直接返回false，对于正数，假设输入的int是一个palindrome，reverse以后依然不会溢出，因此正常返回true；因此若是转换后溢出了，证实确定不是palindrome，溢出后的数字跟输入通常不相同（想不出相等的状况-_-!，求指点），因此返回了false。

思路2：从两头依次取数字比较，向中间推动。

public class Solution {
    public boolean isPalindrome(int x) {
        if (x < 0)
            return false;
        //calcu the length of digit
        int len = 1;
        while (x / len >= 10) {
            len *= 10;
        }

        while (x != 0) {
            int left = x / len;
            int right = x % 10;

            if (left != right)
                return false;
            //remove the head and tail digit
            x = (x % len) / 10;
            len /= 100;
        }

        return true;
    }

}

参考：

http://www.programcreek.com/2013/02/leetcode-palindrome-number-java/

http://fisherlei.blogspot.com/2012/12/leetcode-palindrome-number.html