121. Best Time to Buy and Sell Stock

Best Time to Buy and Sell Stock - LeetCode

The main idea is to keep track of the lowest price seen so far.

Intuition

If we want the maximum profit, then for each day we should ask:

If I sold today, what is the best buy price I could have used earlier?

That means we do not need to compare every pair of days. We only need to remember the smallest price we have seen so far as we scan from left to right.

For each current price:

• treat it as a possible selling price
• subtract the minimum earlier price
• update the best profit if needed

This works because the buy must happen before the sell, and scanning left to right naturally preserves that rule.

Implementation

I use:

• min_price to store the cheapest price seen so far
• max_profit to store the best profit found so far

For each price in the list:

• update min_price
• calculate price - min_price
• compare it with max_profit

If prices only go downward, then no profitable trade exists, and max_profit correctly remains 0.

# 121. Best Time to Buy and Sell Stock

def maxProfit(prices):
    min_price = float("inf")
    max_profit = 0

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

    return max_profit


print(maxProfit([7, 1, 5, 3, 6, 4]))