https://brilliant.org/daily-problems/instruction-order/
考察编程思想，通过拖拽图标3步实现任务目标
how to swap x, y ?
'''
Initially, variables x and y contain values A and B, respectively.
You have three instructions at your disposal:
a. Set x to x minus y.
b. Set x to x plus y.
c. Set y to x minus y.
x, y = 3, 4
x, y = 3, 4
x = x + y
y = x - y
x = x - y
print(x,y)
x, y = 4, 3
