You are given an array `prices`

where `prices[i]`

is the price of a given stock on the `i`

day.^{th}

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`

.

**Example 1:**

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

**Example 2:**

Input:prices = [7,6,4,3,1]Output:0Explanation:In this case, no transactions are done and the max profit = 0.

**Constraints:**

`1 <= prices.length <= 10`

^{5}`0 <= prices[i] <= 10`

^{4}

```
struct Solution;
impl Solution {
fn max_profit(prices: Vec<i32>) -> i32 {
let mut cost = std::i32::MAX;
let mut profit = 0;
for price in prices {
profit = profit.max(price - cost);
cost = cost.min(price);
}
profit
}
}
#[test]
fn test() {
let prices = vec![7, 1, 5, 3, 6, 4];
let res = 5;
assert_eq!(Solution::max_profit(prices), res);
let prices = vec![7, 6, 4, 3, 1];
let res = 0;
assert_eq!(Solution::max_profit(prices), res);
}
```