题目描述：
At a lemonade stand, each lemonade costs $5.
Customers are standing in a queue to buy from you, and order one at a time (in the order specified by bills).
Each customer will only buy one lemonade and pay with either a $5, $10, or $20 bill. You must provide the correct change to each customer, so that the net transaction is that the customer pays $5.
Note that you don't have any change in hand at first.
Return true if and only if you can provide every customer with correct change.
Example 1:
Input: [5,5,5,10,20] Output: true Explanation: From the first 3 customers, we collect three $5 bills in order. From the fourth customer, we collect a $10 bill and give back a $5. From the fifth customer, we give a $10 bill and a $5 bill. Since all customers got correct change, we output true.
Example 2:
Input: [5,5,10] Output: true
Example 3:
Input: [10,10] Output: false
Example 4:
Input: [5,5,10,10,20] Output: false Explanation: From the first two customers in order, we collect two $5 bills. For the next two customers in order, we collect a $10 bill and give back a $5 bill. For the last customer, we can't give change of $15 back because we only have two $10 bills. Since not every customer received correct change, the answer is false.
解题思路
贪心选择策略很简单，就是记录当前还剩的5元、10元的零钱数量（注意，这里有一个小trick，我们不需要记录20元钱的数量，因为20元只会收入，不会在找零的过程中给出去）。
Python解法
代码还有点乱。。
class Solution:
def lemonadeChange(self, bills):
"""
:type bills: List[int]
:rtype: bool
"""
if not bills:
return True
a, b = 0, 0
for x in bills:
if x == 5:
a += 1
if x == 10:
a -= 1
b += 1
if x == 20:
if b > 0:
b -= 1
a -= 1
else:
a -= 3
if (a < 0) | (b < 0):
return False
return True