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#121EasySliding window

Best Time to Buy and Sell Stock

The tell: Best single buy-then-sell pair, where the buy must precede the sell.

How you get there

  1. 1Track the cheapest price seen so far as you sweep forward.
  2. 2At each day the best sale today is price − cheapest-so-far.
  3. 3One pass, no need to consider pairs explicitly.

Solution

Python
def max_profit(prices: list[int]) -> int:
    cheapest = float("inf")
    best = 0
    for price in prices:
        cheapest = min(cheapest, price)
        best = max(best, price - cheapest)
    return best
Time
O(n)
Space
O(1)

What goes wrong

Updating the answer before the minimum allows selling on the same day you buy — harmless here, wrong in variants.

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