Leetcode: 9. Palindrome Number

Description

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

Example

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

• 负数都不是回文数
• 先计算位数
• 然后在把前后分成两个数，判断是否相等

代码

• 时间复杂度：O(m) m为位数

class Solution {
public:
    bool isPalindrome(int x) {
        if(x < 0)
           return false;
        
        int count = 0, tmp = x;
        while(tmp){
            count++;
            tmp /= 10;
        }
        tmp = x;
        
        int sum = 0;
        int num = count / 2;
        while(num){
            sum = sum * 10 + (tmp % 10);
            tmp /= 10;
            num--;
            x /= 10;
        }
        
        if(count & 1) x /= 10;
        
        return sum == x;
    }
};