860. Lemonade Change

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.

Rust Solution

struct Solution;

impl Solution {
    fn lemonade_change(bills: Vec<i32>) -> bool {
        let mut b5 = 0;
        let mut b10 = 0;
        for b in bills {
            match b {
                5 => {
                    b5 += 1;
                }
                10 => {
                    if b5 > 0 {
                        b10 += 1;
                        b5 -= 1;
                    } else {
                        return false;
                    }
                }
                20 => {
                    if b10 > 0 && b5 > 0 {
                        b10 -= 1;
                        b5 -= 1;
                    } else if b5 >= 3 {
                        b5 -= 3;
                    } else {
                        return false;
                    }
                }
                _ => {}
            }
        }
        true
    }
}

#[test]
fn test() {
    let bills = vec![5, 5, 5, 10, 20];
    assert_eq!(Solution::lemonade_change(bills), true);
    let bills = vec![5, 5, 10];
    assert_eq!(Solution::lemonade_change(bills), true);
    let bills = vec![10, 10];
    assert_eq!(Solution::lemonade_change(bills), false);
    let bills = vec![5, 5, 10, 10, 20];
    assert_eq!(Solution::lemonade_change(bills), false);
    let bills = vec![
        5, 5, 10, 20, 5, 5, 5, 5, 5, 5, 5, 5, 5, 10, 5, 5, 20, 5, 20, 5,
    ];
    assert_eq!(Solution::lemonade_change(bills), true);
}

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