Power of leetcode-231-2

Title Description

Given an integer , Write a function to determine if it is 2 Power square .

Their thinking

Determine whether an integer is 2 Power square , You can use 2 Properties of the power of , Turn numbers into binary .
We know , If a number n yes 2 Power square , Then its binary representation must first be 1, The rest are 0. and n-1 In the binary representation of , The first became 0, The rest are 1.
for example , The figure below represents n=16 Binary representation of time ：

……	$2^4$	$2^3$	$2^2$	$2^1$	$2^0$
n=16	1	0	0	0	0
n=15	0	1	1	1	1

& The characteristic of operation is , Same as 1, Different for 0
In this case ,n and n-1 Do and calculate （&）, Everyone is 0&1, The result must be 0
That is to say if n yes 2 Power square , be n&(n-1)=0
If n No 2 Power square , The result is greater than 0
therefore , utilize & To judge by operation

Code

Code 1 ： Forward calculation
Stick me first 2min Code completed within （laji code

import math
def isPowerOfTwo(self, n):
    p = 0
    if n <= 0:
        return False
    while True:
        num = math.pow(2, p)     
            
        if n < num:
            return False
        elif n == num:
            return True
        else:
            p += 1
            continue

Code 2 ： Reverse calculation

def isPowerOfTwo(self, n):
    if n:
        while n % 2 == 0 :
            n /= 2
        return n == 1
    return False

Code one is as efficient as code two （ As low as ）

Code three ： Someone else's code

def isPowerOfTwo(self, n):
    if n < 1:
        return False
    return not (n & (n-1))

Simple and clear , Efficient and comfortable .