Leetcode 0860. 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.

Easy

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 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

• 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

• 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 <= 10^5
• bills[i] is either 5, 10, or 20.

Solution

class Solution {
  /**
   * Iteration with using integer variables
   * Time Complexity: BigO(n)
   * Space Complexity: BigO(1)
   */
  public boolean lemonadeChange(int[] bills) {
    int b5 = 0;
    int b10 = 0;

    for (int bill : bills) {
      if (bill == 5) b5++;
      else if (bill == 20){
         if (b5 >= 1 && b10 >= 1) {
           b10--;
           b5--;
           continue;
         }
         else if (b5 >= 3 && b10 == 0) {
          b5 -= 3;
          continue;
         }
         return false;
      }
      else if (bill == 10) {
        if (b5 == 0) return false;
        b10++;
        b5--;
      }
    }
    return true;
  }
}