檸檬水??找零問題，有5 10 20三種面值。解法：能用10元找零就不用5塊。

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.
 

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int five=0,ten=0;
        for(int i:bills){
            if(i==5){
                five++;
            }
            else if(i==10){
                if(five>=1){
                    ten++;
                    five--;
                }
                else{
                    return false;
                }
            }
            else if(i==20){
                if(ten>=1&&five>=1){
                    ten--;
                    five--;
                }else if(five>=3){
                    five-=3;
                }else{
                    return false;
                }
            }
        }
        return true;
    }
};

python代碼

class Solution(object):
    def lemonadeChange(self, bills):
        """
        :type bills: List[int]
        :rtype: bool
        """
        five,ten=0,0
        for bill in bills:
            #print "bill: {}".format(bill)
            if bill==5:
                five+=1
            elif bill==10:
                if five>=1:
                    five-=1
                    ten+=1
                else:
                    return False
            elif bill==20:
                if ten>=1 and five>=1:
                    ten-=1
                    five-=1
                elif five>=3:
                    five-=3
                else:
                    return False
            #print "five: {}, ten: {}".format(five,ten)
                
        return True
                    
        

860. Lemonade Change