860. Lemonade Change

Description:

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.

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

'''
[1] Greedy)

idea::
1. keep the number of five dollars and ten dollars.

code::
(below)


-T/C: O(n) # n==len(bills)
-S/C: O(1)
'''

class Solution:
    def lemonadeChange(self, bills: List[int]) -> bool:
        five=0
        ten=0
#       twenty=0
        
        for bill in bills:
            if bill==5:
                five+=1
            elif bill==10:
                if five<1:
                    return False
                ten+=1
                five-=1
            else:
                if five>=1 and ten>=1:
                    ten-=1
                    five-=1
                elif five>=3:
                    five-=3
                else:
                    return False
                twenty+=1
                
        return True