leetcode-860-easy

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.

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.

思路一：完全分类，统计 5 和 10 的个数

    public boolean lemonadeChange(int[] bills) {
        int[] count = new int[2];

        for (int bill : bills) {
            int val = bill - 5;
            if (val == 5) {
                if (count[0] > 0) {
                    count[0]--;
                } else {
                    return false;
                }
            } else if (val == 15) {
                if (count[1] > 0 && count[0] > 0) {
                    count[1]--;
                    count[0]--;
                } else if (count[1] == 0 && count[0] > 2) {
                    count[0] -= 3;
                } else {
                    return false;
                }
            }

            if (bill == 5) count[0]++;
            if (bill == 10) count[1]++;
        }

        return true;
    }