860. Lemonade Change

QUESTION:

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.

EXPLANATION:

采用的是贪心算法，首先需要记录的点是，当前有多少的零钱。那么就可以采用数组或者直接用数字来标识剩余的钱。

情况就可以分为3种：

1.收到5元，那么就直接收下。

2.收到19元，找回5元

3.收到20，找零10+5 或者是5+5+5

采用贪心算法，如果有10元那么就使用10元的。

SOLUTION:

class Solution {
    public boolean lemonadeChange(int[] bills) {
        int[] cash = new int[3];
        for(int i=0;i<bills.length;i++){
            switch (bills[i]){
                case 5:
                    cash[0]++;
                    break;
                case 10:
                    cash[0]--;
                    cash[1]++;
                    break;
                case 20:
                    if(cash[1]>0&&cash[0]>0){
                        cash[1]--;
                        cash[0]--;
                    }else {
                        cash[0]-=3;
                    }
                    cash[2]++;
                    break;
            }
            if(!lemonadeChangeHelper(cash)) return false;
        }
        return true;
    }
    
        public static boolean lemonadeChangeHelper(int[] cash){
        for (int item:cash) {
            if(item<0) return false;
        }
        return true;
    }
}
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