One Character Of Difference

This is the USACO Guide solution for nearest smaller element. It reports, for each position, the one-based index of the nearest strictly smaller element to the left, or 0 when there is none.

for idx, num in enumerate(nums):
    while stack and stack[-1][0] >= num:
        stack.pop()
    ans.append(0 if not stack else stack[-1][1] + 1)
    stack.append((num, idx))

A reviewer changes >= to > and claims it is equivalent. Run both on nums = [2, 5, 5, 3]. What actually happens?

Nothing changes; the two conditions differ only when values are equal, and equal values are interchangeable as answers

The third output changes from 1 to 2, and the > version now reports the nearest element to the left that is smaller or equal

The third output changes from 1 to 2, and the > version now reports the nearest strictly smaller element while >= reported smaller-or-equal

The fourth output changes from 1 to 3, because with > the equal value survives on the stack and becomes the answer for the final element