The Runtime Theory
easyLeetCode#running-minimum#one-pass

Best Time to Buy and Sell Stock

Choose one buy day and a later sell day to maximize positive profit; return zero if no profitable trade exists.

The Runtime Theory Team25 min read
Solve it

Solving happens on the judge — come back and mark it done

Sample cases

inprices=[7,1,5,3,6,4]

out5

inprices=[7,6,4,3,1]

out0

inprices=[2,4,1]

out2

Choose one buy day and a later sell day to maximize positive profit; return zero if no profitable trade exists.

A good solution should

  • State the invariant or decision rule that makes the approach correct.
  • Explain the time and space complexity in terms of the input sizes and required output.
  • Handle empty, minimal, duplicate, and boundary-shaped inputs covered by the contract.

Reasoning prompt

Track the cheapest earlier price as you scan. Do not allow the selling day to precede the buying day.

Use the linked judge for the canonical problem. The examples above are compact TRT checks; write additional cases before submitting, especially for the boundary that tends to break your chosen invariant.

More practice in this topic

medium · sliding windowLongest Repeating Character Replacementmedium · sliding windowLongest Substring Without Repeating Charactershard · sliding windowMinimum Window Substring

One dispatch a week

The trace behind each problem, the tradeoff that explains it, and one technical dispatch per week — no noise.

One technical dispatch per week. No noise.

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