Leetcode 数 Palindrome Number

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Palindrome Number

Total Accepted: 12165 Total
Submissions: 41736

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.

题意：判断给定的数字是不是回文，不可以用额外空间

思路：

取首位取末位，检查是否相同。

如果不同，返回false

如果相同，去掉首位去掉末位，继续上面的操作

复杂度：时间O(log n)， 空间O(1)

注：对于像102201这种中间有零的情况，因为x等于220的时候，n还是1000，所以x / n还是能得到220前面的0

相关题目：Reverse Integer

class Solution {
public:
	bool isPalindrome(int x) {
		if(x < 0) return false;
		int n = 1;
		while(x / n >= 10){
			n *= 10;
		}
		while(x){
			int a = x / n;   //取首位数字
			int b = x % 10;  //取末位数字
			if(a != b) return false;
			x %= n ; //去掉首位数字
			x /= 10; //去掉末位数字
			n /= 100; //n要对应去掉两位
		}
		return true;
	}
};

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