leetcode-231-2的幂

题目描述：

第一次提交：

class Solution:
    def isPowerOfTwo(self, n: int) -> bool:
        if n==0:return False
        if n==1:
            return True
        s = str(bin(n))[3:]
        for i in s:
            if i == '1':
                return False
        return True

优化：

class Solution:
    def isPowerOfTwo(self, n: int) -> bool:
        return n > 0 and bin(n).count("1") == 1

方法二：O(1)

class Solution:
    def isPowerOfTwo(self, n: int) -> bool:
        return n > 0 and n & (n - 1) == 0

方法三：O(logn)

class Solution:
    def isPowerOfTwo(self, n: int) -> bool:
        if n == 0: return False
        while n % 2 == 0:
            n //= 2
        return n == 1