9-Palindrome Number

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题意

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward. Follow up: Could you solve it without converting the integer to a string?

Example 1:

• Input: x = 121
• Output: true

Example 2:

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

Example 4:

• Input: x = -101
• Output: false

Constraints:-231 <= x <= 231 - 1

思路1

• 数字转成str
• 判断正向==反向

  思路2

• 手动计算reverse
• 数位反转模板

  reverse = 0
  while x > 0:
    reverse = reverse*10 + x % 10
    x = x/10
  

# Solution 1
class Solution(object):
    def isPalindrome(self, x):
        """
        :type x: int
        :rtype: bool
        """
        return str(x) ==  str(x)[::-1]
        
# Solution 2  
class Solution(object):
    def isPalindrome(self, x):
        """
        :type x: int
        :rtype: bool
        """
        if x < 0: return False
        original = x
        reverse = 0
        while x > 0:
            reverse = reverse*10 + x % 10
            x = x/10
        return reverse == original

分析:

• Time: O(n)
• Space: O(1)