Easy
Sliding Window
Best Time to Buy and Sell Stock
Track the cheapest price so far and the best profit against it.
Approach
Sweep once, keeping the minimum price seen. At each day, the best sale is today's price minus that running minimum; keep the largest such difference. It's a one-pass sliding view of 'buy low before, sell here'.
Time complexity
O(n)
Space complexity
O(1)
Common mistake
Comparing every buy/sell pair (O(n²)) instead of tracking the running minimum.
See it run, step by step
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