860. Lemonade Change

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.

Note:

0 <= bills.length <= 10000
bills[i] will be either 5, 10, or 20.

class Solution:
    def lemonadeChange(self, bills):
        """
        :type bills: List[int]
        :rtype: bool
        """
        a,b = 0,0
        for i in bills:
            if i == 5:
                a += 1
                continue
            if i ==10:
                a -= 1
                if a<0:
                    return False
                b += 1
            if i == 20:
                if b>0:
                    b -= 1
                    a -= 1
                    if a<0 or b<0:
                        return False
                else:
                    a -= 3
                    if a<0:
                        return False
        return True