860. Lemonade Change

1. 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.

2. Example

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.

3. Constraints

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

4. Solutions

Hash Table

n = bills.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    bool lemonadeChange(const vector<int> &bills) {
        unordered_map<int, int> bill_count;
        for (int i = 0; i < bills.size(); ++i) {
            switch (bills[i]) {
            case 5:
                ++bill_count[5];
                break;
            case 10:
                if (bill_count[5] > 0) {
                    --bill_count[5];
                    ++bill_count[10];
                } else {
                    return false;
                }
                break;
            case 20:
                // no need to increase bill_count[20]
                if (bill_count[10] > 0 && bill_count[5] > 0) {
                    --bill_count[5];
                    --bill_count[10];
                } else if (bill_count[5] >= 3) {
                    bill_count[5] -= 3;
                } else {
                    return false;
                }
                break;
            }
        }

        return true;
    }
};