Introduction

In this blog post, we will explore how to solve the LeetCode problem "860" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

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.

Explanation

Here's the breakdown of the approach, complexity, and the Python code:

• High-Level Approach:

  • Maintain a count of $5 and $10 bills we have.
  • Iterate through the bills array. For each bill, check if we can provide the correct change. If not, return False.
  • If we successfully provide change to all customers, return True.

• Complexity:

  • Runtime Complexity: O(n), where n is the number of customers (length of bills).
  • Storage Complexity: O(1), as we only store the counts of $5 and $10 bills.

Code

    def lemonadeChange(bills):
    """
    Determines if you can provide the correct change to every customer at a lemonade stand.

    Args:
        bills (list of int): An array where bills[i] is the bill the ith customer pays.

    Returns:
        bool: True if you can provide change to every customer, False otherwise.
    """

    five_dollar_bills = 0
    ten_dollar_bills = 0

    for bill in bills:
        if bill == 5:
            five_dollar_bills += 1
        elif bill == 10:
            if five_dollar_bills > 0:
                five_dollar_bills -= 1
                ten_dollar_bills += 1
            else:
                return False
        else:  # bill == 20
            if ten_dollar_bills > 0 and five_dollar_bills > 0:
                ten_dollar_bills -= 1
                five_dollar_bills -= 1
            elif five_dollar_bills >= 3:
                five_dollar_bills -= 3
            else:
                return False

    return True