860. 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 don't have any change in hand at first.

Return true if and only if you can provide every customer with correct change.

Example 1:

Input: [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: [5,5,10]
Output: true

Example 3:

Input: [10,10]
Output: false

Example 4:

Input: [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't give change of $15 back because we only have two $10 bills.
Since not every customer received correct change, the answer is false.

Note:

• 0 <= bills.length <= 10000
• bills[i] will be either 5, 10, or 20.

class Solution {
    public boolean lemonadeChange(int[] bills) {
        int rem5 = 0, rem10 = 0;
        for(int i = 0; i < bills.length; i++) {
            int cur = bills[i];
            if(cur == 5) rem5++;
            else if(cur == 10) {
                rem10++;
                if(--rem5 < 0) return false;
            }
            else if(cur == 20) {
                if(rem5 == 0) return false;
                if(rem10 == 0) {
                    if(rem5 < 3) return false;
                    else rem5 -= 3;
                }
                else {
                    --rem10;
                    --rem5;
                }
            }
        }
        return true;
    }
}

simulation, 注意15 = 3 * 5/ 10 + 5