Description

LINK: https://leetcode.com/problems/palindrome-number

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:

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Input: 121
Output: true

Example 2:

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Input: -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

Example 3:

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Input: 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Follow up:

Coud you solve it without converting the integer to a string?

Solution

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static auto _ = []()
{
    ios::sync_with_stdio(false);
    cin.tie();
    return 0;
}();

class Solution {
public:
    bool isPalindrome(int x) {
        if (x == 0) return true;
        if (x < 0 || x % 10 == 0) return false;
        int r = 0;
        while (x > r){
            r = r * 10 + x % 10;
            x = x / 10;
        }
        return r == x || r / 10 == x;
    }
};

Discussion

一般方法是，把整数反过来，然后比较一下。而且，无需判断越界，因为既然输入没有越界，那么如果输出越界，则肯定不是回文数。

改进的方法是，只反转一半，然后跟剩下的一半进行比较。这样可以节省一半运算量。不过并不本质，因为最多也就进行10次运算吧。。。