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Lemonade Change⚓︎

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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 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:
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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:
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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⚓︎

See reference (Chinese).

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class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int five = 0, ten = 0;
        for (int bill : bills) {
            if (bill == 5) five++;
            else if (bill == 10) {
                if (five == 0) return false;
                five--, ten++;
            } else {  // bill == 20
                if (five > 0 && ten > 0) {
                    five--, ten--;
                } else if (five >= 3) {
                    five -= 3;
                } else return false;
            }
        }
        return true;
    }
};