lemonade-change.java

// 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.
//
//


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