> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Lemonade Change Python Solution with Tests

> Tested Python solution for LeetCode 860 with 16 pytest cases. Generate a practice environment with lcpy.

LeetCode 860, Easy. Topics: Array, Greedy. [View on LeetCode](https://leetcode.com/problems/lemonade-change/description/).

Generate this problem as a practice environment: tested reference solution, 16 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 860   # by problem number
lcpy gen -s lemonade_change   # by problem name
```

## Problem

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 do not have any change in hand at first.

Given an integer array `bills` where `bills[i]` is the bill the `ith` customer pays, return `true` if you can provide every customer with the correct change, or `false` otherwise.

### Examples

```
Input: bills = [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.
```

```
Input: bills = [5,5,10,10,20]
Output: false
```

### Constraints

* 1 \<= bills.length \<= 10^5
* `bills[i]` is either `5`, `10`, or `20`.

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/lemonade_change/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/lemonade_change/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def lemonade_change(self, bills: list[int]) -> bool:
        fives = 0
        tens = 0
        for bill in bills:
            if bill == 5:
                fives += 1
            elif bill == 10:
                if fives == 0:
                    return False
                fives -= 1
                tens += 1
            else:  # bill == 20, prefer giving a $10 + $5 to conserve $5 bills
                if tens > 0 and fives > 0:
                    tens -= 1
                    fives -= 1
                elif fives >= 3:
                    fives -= 3
                else:
                    return False
        return True
```

## Complexity

| Time | Space |
| ---- | ----- |
| O(n) | O(1)  |

## Tags

[NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).