Maximum Profit From a Single Buy Followed by a Later Sell
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Onsite
You are given the price of one stock on each of a sequence of days: `prices[i]` is the price on day `i`. You may buy one share on some day and sell it on a strictly later day, at most once. Return the largest profit you can make. If no buy-then-sell pair makes money, you do not trade and the profit is 0.
### Function Signature
```python
def max_single_trade_profit(prices: list[int]) -> int:
```
### Rules
- Buying on day `i` and selling on day `j` earns `prices[j] - prices[i]`, and requires `i < j`. You cannot sell before you buy.
- At most one buy and one sell in total. Not trading is always allowed, so the answer is never negative.
- Return only the profit, which is unique even when several pairs of days achieve it.
### Constraints
- `1 <= len(prices) <= 100000`
- `0 <= prices[i] <= 10000`
- The answer is an integer between 0 and 10000.
### Examples
**Example 1**
```text
Input: prices = [9, 4, 6, 3, 8, 5]
Output: 5
```
Buy on day 3 at price 3 and sell on day 4 at price 8. Buying at 4 and selling at 8 would earn only 4.
**Example 2**
```text
Input: prices = [10, 8, 5, 5, 1]
Output: 0
```
The price never rises from any day to a later day, so the best choice is not to trade.
**Example 3**
```text
Input: prices = [7]
Output: 0
```
With a single day there is no later day to sell on.
Overview: An Amazon coding problem: given daily stock prices, find the maximum profit from buying once and selling on a later day, or zero if no trade makes money. It tests linear-time reasoning over an array and careful handling of falling prices and single-day input.
Read the full Amazon Software Engineer interview experience this question came from
