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 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.

 */

public class Lemonade_Change {

    class Solution {

        int count5 = 0;
        int count10 = 0;
        int count20 = 0;

        public boolean lemonadeChange(int[] bills) {

            if (bills == null || bills.length == 0) {
                return false;
            }

            for (int i = 0; i < bills.length; i++) {

                if (bills[i] == 5) {
                    count5++;
                } else if (bills[i] == 10) {
                    if (count5 > 0) {
                        count5--;
                        count10++;
                    } else {
                        return false;
                    }
                } else if (bills[i] == 20) {
                    if (count5 > 0 && count10 > 0) {
                        count5--;
                        count10--;
                        count20++; // not used, but still record it
                    } else if (count5 > 2) {
                        count5 -= 3;
                    } else {
                        return false;
                    }
                }
            }

            return true;
        }
    }
}

// OJ: https://leetcode.com/problems/lemonade-change/
// Time: O(N)
// Space: O(1)
class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int five = 0, ten = 0;
        for (auto b : bills) {
            if (b == 5) {
                ++five;
            } else if (b == 10) {
                if (!five) return false;
                --five;
                ++ten;
            } else {
                if (five && ten) {
                    --five;
                    --ten;
                } else if (five >= 3) {
                    five -= 3;
                } else {
                    return false;
                }
            }
        }
        return true;
    }
};

class Solution:
    def lemonadeChange(self, bills: List[int]) -> bool:
        five = ten = 0
        for v in bills:
            if v == 5:
                five += 1
            elif v == 10:
                ten += 1
                five -= 1
            else:
                if ten:
                    ten -= 1
                    five -= 1
                else:
                    five -= 3
            if five < 0:
                return False
        return True

############
3
class Solution:
    def lemonadeChange(self, bills):
        """
        :type bills: List[int]
        :rtype: bool
        """
        changes = {5:0, 10:0}
        for bill in bills:
            if bill == 5:
                changes[5] += 1
            elif bill == 10:
                if changes[5] == 0:
                    return False
                else:
                    changes[10] += 1
                    changes[5] -= 1
            elif bill == 20:
                if changes[10] != 0:
                    if changes[5] == 0:
                        return False
                    else:
                        changes[5] -= 1
                        changes[10] -= 1
                else:
                    if changes[5] < 3:
                        return False
                    else:
                        changes[5] -= 3
        return True

Java

class Solution {
    public boolean lemonadeChange(int[] bills) {
        int count5 = 0, count10 = 0;
        int length = bills.length;
        for (int i = 0; i < length; i++) {
            int bill = bills[i];
            if (bill == 5)
                count5++;
            else if (bill == 10) {
                count10++;
                if (count5 == 0)
                    return false;
                else
                    count5--;
            } else if (bill == 20) {
                if (count10 == 0) {
                    if (count5 < 3)
                        return false;
                    else
                        count5 -= 3;
                } else {
                    if (count5 == 0)
                        return false;
                    else {
                        count10--;
                        count5--;
                    }
                }
            }
        }
        return true;
    }
}

// OJ: https://leetcode.com/problems/lemonade-change/
// Time: O(N)
// Space: O(1)
class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int five = 0, ten = 0;
        for (auto b : bills) {
            if (b == 5) {
                ++five;
            } else if (b == 10) {
                if (!five) return false;
                --five;
                ++ten;
            } else {
                if (five && ten) {
                    --five;
                    --ten;
                } else if (five >= 3) {
                    five -= 3;
                } else {
                    return false;
                }
            }
        }
        return true;
    }
};

class Solution:
    def lemonadeChange(self, bills: List[int]) -> bool:
        five = ten = 0
        for v in bills:
            if v == 5:
                five += 1
            elif v == 10:
                ten += 1
                five -= 1
            else:
                if ten:
                    ten -= 1
                    five -= 1
                else:
                    five -= 3
            if five < 0:
                return False
        return True

############
3
class Solution:
    def lemonadeChange(self, bills):
        """
        :type bills: List[int]
        :rtype: bool
        """
        changes = {5:0, 10:0}
        for bill in bills:
            if bill == 5:
                changes[5] += 1
            elif bill == 10:
                if changes[5] == 0:
                    return False
                else:
                    changes[10] += 1
                    changes[5] -= 1
            elif bill == 20:
                if changes[10] != 0:
                    if changes[5] == 0:
                        return False
                    else:
                        changes[5] -= 1
                        changes[10] -= 1
                else:
                    if changes[5] < 3:
                        return False
                    else:
                        changes[5] -= 3
        return True

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