0860. Lemonade Change

https://leetcode.com/problems/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.

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

Example 3:

**Input:** bills = [5,5,10]
**Output:** true

Example 4:

**Input:** bills = [10,10]
**Output:** false

Constraints:

• 1 <= bills.length <= 105

• bills[i] is either 5, 10, or 20.

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