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 do not have any change in hand at first.

Given an integer array bills where bills[i] is the bill the ith customer pays, return true if you can provide every customer with the correct change, or false otherwise.

 

Example 1:

Input: bills = [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: bills = [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 not give the change of $15 back because we only have two $10 bills.
Since not every customer received the correct change, the answer is false.

 

Constraints:

• 1 <= bills.length <= 105
• bills[i] is either 5, 10, or 20.
•  

---

def lemonadeChange(self, bills: List[int]) -> bool:
        adict={5:0,10:0,20:0}
        for i in bills:
            adict[i]+=1
            if i==10:
                if adict[5]==0:
                    return False
                adict[5]-=1
            if i==20:
                if adict[10]==0:
                    if adict[5]>=3:
                        adict[5]-=3
                    else:
                        return False
                else:
                    adict[10]-=1
                    if adict[5]>=1:
                        adict[5]-=1
                    else:
                        return False
        return True