9 Palindrome Number 回文数
Description:
Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.
Example:
Example 1:
Input: 121
Output: true
Example 2:
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:
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?
题目描述:
判断一个整数是否是回文数。回文数是指正序（从左向右）和倒序（从右向左）读都是一样的整数。
示例:
示例 1:
输入: 121
输出: true
示例 2:
输入: -121
输出: false
解释: 从左向右读, 为 -121 。 从右向左读, 为 121- 。因此它不是一个回文数。
示例 3:
输入: 10
输出: false
解释: 从右向左读, 为 01 。因此它不是一个回文数。
进阶:
你能不将整数转为字符串来解决这个问题吗？
思路:
- 字符串反转
- 反转一半数字, 直到反转数字大于原数字, 最后一位可以忽略
时间复杂度O(lgn), 空间复杂度O(1)
代码:
C++:
class Solution
{
public:
// C++ bool型: true/false
bool isPalindrome(int x)
{
if (x < 0 or x % 10 == 0 and x != 0) return false;
int reverse = 0;
while (x > reverse)
{
reverse = reverse * 10 + x % 10;
x /= 10;
}
return x == reverse or x == reverse / 10;
}
};
Java:
class Solution {
public boolean isPalindrome(int x) {
String xString = String.valueOf(x);
String reverse = new StringBuffer(xString).reverse().toString();
return xString.equals(reverse);
}
}
Python:
class Solution:
def isPalindrome(self, x: int) -> bool:
return str(x) == str(x)[::-1]
