LeetCode860. Lemonade Change

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