Number of Visible People in a Queue

Keep people still visible from the left

Process heights from right to left with a decreasing stack. For the current person, pop every shorter height, counting each because it is directly visible until the current person blocks it from anyone further left. If a taller height remains on top, count that one too: it is visible but blocks everybody behind it. Push the current height.

The stack stores a decreasing skyline of people not yet hidden by someone closer and taller. Every height is pushed once and popped once, so despite the loop, time is O(n) and space is O(n). Test increasing heights, decreasing heights, one person, and the equality rule specified by the prompt; standard variants use distinct heights. A naive scan rightward from each index is O(n²). Be precise that the answer is per person, not merely the total pair count, and that a taller blocker is counted exactly once after shorter people are removed.