EPI300 skyline problem

Sound byte: use SQL

PROBLEM STATEMENT
We have to design a program which helps drawing the skyline of a two-dimensional city given the locations of the rectangular buildings in the city. Each building B_i is represented by a triplet of (L_i, R_i, h_i) where L_i and R_i are the left and right coordinates of the ith building, and h_i is the height. In the diagram below there are 8 buildings, represented from left to right by the triplets (1, 5, 11), (2, 7, 6), (3, 9, 13), (12, 16, 7), (14, 25, 3), (19, 22, 18), (23, 29, 13) and (24, 28, 4).

Input
The input of the program is a sequence of building triplets. The triplets are sorted by L_i (the left coordinate of the building) in ascending order.

Q1 —– For any given x, determine the height in the skyline.
Note If x == R_i, then the ith building doesn’t count. In other words, If you look at the first building, the 1-to-5 range it covers is a half-open interval, sometimes written as [1,5) as this range includes 1 but excludes 5. You can think of [1,5) as [1, 4.99999999] approximately.

A1(brute force): look at each building and decide if it “covers” the point X. Given the pre-sort, most buildings aren’t relevant. A complete solution would be

Select max(h) from myTable t where t.l =< x < t.r

To support this solution, the objects could be stored in two sorted data structures, one sorted by L_i and one sorted by R_i.

Q2 —- draw the skyline.
A2: evaluate Q1 (i.e. get the height) at every L_i and R_i value. This solution is probably suboptimal.

Q2b —- draw the skyline by outputting a sequence of {x,h} pairs showing the change in height (represented by a new height h) at each vertical edge (marked by x).

Is it possible to do this in one scan after preprocessing the triplets with some clever data structures? [[EPI300]] may have a solution. Here’s my proposal —

Sort the N buildings into a list by L_i, and sort the same buildings into another list by R_i, and merge the 2 lists into a big sorted list of 2N objects. Each building shows up twice. Each of the 2N entries consists of {building_object_id, x, boolean flag left_or_right }. We will go through this big list one-pass.

As we hit the left edge of a building, we include this building in the Alive system. In Alive, we keep the buildings sorted by height. We also maintain a lookup table keyed by building_id. As we hit the right edge of a building, we remove it from the lookup table and from Alive.

As we hit any edge, we need to determine the impact on the skyline if any. This is when we make use of the Alive system. For any edge, there’s impact iff the target building is tallest in Alive.

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