860. Lemonade Change

https://leetcode.com/problems/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 510, or 20.
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Intuition

Simulation
    if (bill == 5)
        five++
    else if (bill == 10)
        if (five == 0)
            return false
        else
            five--;
            ten++;
    else
        if (five > 0 && ten > 0)
            five--;
            ten--;
        else if (five >= 3)
            five -=3;
        else
            return false;

return true

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Time - O(N)
Space - O(1)
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