 Similar Problems:

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:

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

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

• Solution:
```// Blog link: https://code.dennyzhang.com/lemonade-change
// Basic Ideas:
//   For \$5, no need for change.
//   For \$20, we can't use it for the following customer
// Complexity: Time O(n), Space O(1)
count_f, count_t := 0, 0
for _, bill := range bills {
if bill == 5 {
count_f++
} else {
if bill == 10 {
if count_f == 0 { return false }
count_f, count_t = count_f-1, count_t+1
} else {
// bill == 20
if count_f == 0 { return false }
count_f--
if count_t != 0 {
count_t--
} else {
if count_f >= 2 {
count_f-=2
} else {
return false
}
}
}
}
}
return true
}
```

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