柠檬水?找零问题，有5 10 20三种面值。解法：能用10元找零就不用5块。

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: trueExplanation: 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: falseExplanation: 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
     
      & 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代码

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