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You may know coding, but can you talk about it?

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A session, with the problem solved

Python

Widest Water Between Posts

Medium
credits 46
solution.py
def solve(heights):
    left, right = 0, len(heights) - 1
    best = 0

    while left < right:
        width = right - left
        best = max(best, width * min(heights[left], heights[right]))

        # The shorter post caps this pair, and every pair still to come
        # behind it is narrower — so it can never beat what we have.
        if heights[left] <= heights[right]:
            left += 1
        else:
            right -= 1

    return best
hints 011:38
You
Two pointers, one at each end. Each step I move the shorter post inward.
Tutor
Why the shorter one? Say what you throw away when you move it.
You
Every pair that used that post. The width only shrinks from here, and that post already caps the height.
Tutor
Good. So the loop is O(n), one pass, no extra space — agreed. Write it.
You
Done. Both samples and all seven hidden tests pass.
Tutor
They do. Harder question, and it is the one an interviewer asks next: what happens on the step where the two posts are exactly equal, and why is it safe to move either?
  • Hidden tests run server-side
  • Difficulty-weighted score
  • Your code carried through the session
  • How it works

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