> ## 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.

# Best Time to Buy and Sell Stock

> Tested Python solution for LeetCode 121 with 15 pytest cases. Generate a practice environment with lcpy.

LeetCode 121, Easy. Topics: Array, Dynamic Programming. [View on LeetCode](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/).

Generate this problem as a practice environment: tested reference solution, 15 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 121   # by problem number
lcpy gen -s best_time_to_buy_and_sell_stock   # by problem name
```

## Problem

You are given an array `prices` where `prices[i]` is the price of a given stock on the ith day.

You want to maximize your profit by choosing a **single day** to buy one stock and choosing a **different day in the future** to sell that stock.

Return *the maximum profit you can achieve from this transaction*. If you cannot achieve any profit, return `0`.

### Examples

```
Input: prices = [7,1,5,3,6,4]
Output: 5
```

**Explanation:** Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.
Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

```
Input: prices = [7,6,4,3,1]
Output: 0
```

**Explanation:** In this case, no transactions are done and the max profit = 0.

### Constraints

* 1 \<= prices.length \<= 10^5
* 0 \<= prices\[i] \<= 10^4

## Solution

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

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def max_profit(self, prices: list[int]) -> int:
        min_price = prices[0]
        max_profit = 0

        for price in prices[1:]:
            max_profit = max(max_profit, price - min_price)
            min_price = min(min_price, price)

        return max_profit
```

## Complexity

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

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [Blind 75](/catalog/blind-75), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).