[leetcode]Palindrome Number

Palindrome Number

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

click to show spoilers.

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.

算法思路：

重点是求出该整数的位数！

public class Solution {
 public boolean isPalindrome(int x) {
        if(x < 0) return false;
        int n = x;
        int length = 1;
        while((n = n / 10) != 0) length++;
        while(length > 1){
            int begin = x / pow(length);
            int end = x % 10;
            if(begin != end) return false;
            x = (x - begin * pow(length)) / 10;
            length -= 2;
        }
        return true;
    }
    private int pow(int n){
        int result = 1;
        for(int i = 1; i < n; result *= 10,i++);
        return result;
    }
}

不过这道题在leetcode上有专题讲解，我不该班门弄斧的。时间复杂度O(length)，空间O(1)