# 常数阶 O(1)
n = 10

# 线性阶 O(n)
for i in range(n):
    print(i)

# 平方阶 O(n^2)
for i in range(n):
    for j in range(n):
        print(i + j)

i = 1
while i <= n:
    i = i * 2
    print(i)

i*2之后，距离n越来越近

输出的i值：
2   4    8       16
2   2*2  2*2*2   2*2*2*2
2^1 2^2  2^3     2^4

假设需要执行x次，那么2^x = n
=> x = log_2(N) 
=> O(logN)