860. Lemonade Change
Difficulty: Easy
Topics: Array
, Greedy
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 i^{th}
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:
 5

bills[i]
is either5
,10
, or20
.
Solution:
We need to simulate the process of providing change to customers based on the bills they use to pay. The key is to track the number of $5 and $10 bills you have, as these are needed to provide change for larger bills
Let's implement this solution in PHP: 860. Lemonade Change
Explanation:
Initialization: We start with
$five
and$ten
set to 0, representing the count of $5 and $10 bills we have.
Processing Each Bill:
 If the customer pays with a $5 bill: We simply increment the count of $5 bills.

If the customer pays with a $10 bill: We need to give back one $5 bill as change, so we decrement the count of $5 bills and increment the count of $10 bills. If we don't have any $5 bills, return
false
. 
If the customer pays with a $20 bill: We prioritize giving one $10 bill and one $5 bill as change. If that's not possible, we try to give three $5 bills. If neither option is available, return
false
.
Final Check: If we've successfully processed all customers without running out of change, return
true
.
Edge Cases:
 The function should handle scenarios where it's impossible to give the correct change, such as when you receive a $10 or $20 bill too early without having the necessary $5 bill(s) on hand.
 It should efficiently handle large input sizes due to the constraints (up to 100,000 customers). The solution runs in O(n) time complexity, making it optimal for this problem.
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