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# 0122. Best Time to Buy and Sell Stock II

<https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii>

## Description

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

On each day, you may decide to buy and/or sell the stock. You can only hold **at most one** share of the stock at any time. However, you can buy it then immediately sell it on the **same day**.

Find and return *the **maximum** profit you can achieve*.

**Example 1:**

```
**Input:** prices = [7,1,5,3,6,4]
**Output:** 7
**Explanation:** Buy on day 2 (price = 1) and sell on day 3 (price = 5), profit = 5-1 = 4.
Then buy on day 4 (price = 3) and sell on day 5 (price = 6), profit = 6-3 = 3.
Total profit is 4 + 3 = 7.
```

**Example 2:**

```
**Input:** prices = [1,2,3,4,5]
**Output:** 4
**Explanation:** Buy on day 1 (price = 1) and sell on day 5 (price = 5), profit = 5-1 = 4.
Total profit is 4.
```

**Example 3:**

```
**Input:** prices = [7,6,4,3,1]
**Output:** 0
**Explanation:** There is no way to make a positive profit, so we never buy the stock to achieve the maximum profit of 0.
```

**Constraints:**

* `1 <= prices.length <= 3 * 104`
* `0 <= prices[i] <= 104`

## ac1: one pass

very smart solution

```java
class Solution {
    public int maxProfit(int[] prices) {
        // edge case
        if (prices == null || prices.length <= 1) return 0;

        // walk through
        int profit = 0;
        for (int i = 0; i < prices.length - 1; i++) {
            if (prices[i+1] > prices[i]) {
                profit += prices[i+1] - prices[i];
            }
        }

        return profit;
    }
}
/*
[7, 1, 5, 3, 6, 4]
buy point, valley
sell point, peak

*/
```

![](https://leetcode.com/media/original_images/122_maxprofit_2.PNG)

## ac2: find peak and valley

```java
class Solution {
    public int maxProfit(int[] prices) {
        // edge case
        if (prices == null || prices.length <= 1) return 0;

        // walk through
        int profit = 0;
        for (int i = 0; i < prices.length - 1; i++) {
            while (i < prices.length - 1 && prices[i] >= prices[i+1]) i++;  // find buy point
            int buy = prices[i];
            while (i < prices.length - 1 && prices[i] <= prices[i+1]) i++;  // find sell point;
            int sell = prices[i];
            profit += sell - buy;
        }

        return profit;
    }
}
/*
[7, 1, 5, 3, 6, 4]
buy point, valley
sell point, peak

*/
```