Day 4 - Best Time to Buy and Sell a Stock

🔍 Examples:

Example 1
Input: prices = [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1), sell on day 5 (price = 6). Profit = 6 - 1 = 5.

Example 2
Input: prices = [7,6,4,3,1]
Output: 0
Explanation: No opportunity to make a profit.

📏 Constraints:

• 1 <= prices.length <= 10⁵

• 0 <= prices[i] <= 10⁴

🧠 How to Approach It

This one’s a classic example of a one-pass greedy solution.

You’re tracking two things:

1. The lowest price seen so far (min_price)

2. The maximum profit you could make if you sold at the current price

As you walk through the array:

• If today’s price is lower than min_price, update min_price

• Otherwise, calculate price - min_price and see if it’s your best profit yet

What to watch out for: You can’t sell something before you buy it!

✅ Python Solution:

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        min_price = float('inf')
        max_profit = 0

        for price in prices:
            if price < min_price:
                min_price = price
            elif price - min_price > max_profit:
                max_profit = price - min_price

        return max_profit

🧵 TL;DR:

• 📉 Track the lowest price seen so far

• 💸 Compare each day’s price to that low

• ⚡ O(n) time, O(1) space, elegant and fast

• 📉📈 One good trade is all you need