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.

Note:

1. 0 <= bills.length <= 10000
2. bills[i] will be either 5, 10, or 20.

1. 一个物品5美元，每个人依次来买，且只买一个
2. 你初始没钱，要保证每个顾客你都能立即完成找钱操作
3. 顾客给的钱只会是5,10,20，数据长度达到10000，说明O(n^2)复杂度不可行

1. 如果顾客给的是5元，那么不用做任何操作，直接收下即可
2. 如果顾客给的是10元，那么需要找一个5元
3. 如果顾客给的是20元，那么我们有两种选择，一种是找一个5元和一个10元，一种是找三个5元，那么显然因为5元更加灵活（可以用来应对10元情况），所以我们应当优先用第一种找钱方法。
4. 用一个字典记录5元，10元的个数，当剩余钱数不够找即return False!