Approach Framework

Intuition

Intuitively, there are two operations: update, which updates our notion of the board (number line) after dropping a square; and query, which finds the largest height in the current board on some interval. We will work on implementing these operations.

Coordinate Compression

In the below approaches, since there are only up to 2 * len(positions) critical points, namely the left and right edges of each square, we can use a technique called coordinate compression to map these critical points to adjacent integers, as shown in the code snippets below.

For brevity, these snippets are omitted from the remaining solutions.

Approach 1: Offline Propagation

Intuition

Instead of asking the question "what squares affect this query?", lets ask the question "what queries are affected by this square?"

Algorithm

Let qans[i] be the maximum height of the interval specified by positions[i]. At the end, we'll return a running max of qans.

For each square positions[i], the maximum height will get higher by the size of the square we drop. Then, for any future squares that intersect the interval [left, right) (where left = positions[i], right = positions[i] + positions[i]), we'll update the maximum height of that interval.

Complexity Analysis

• Time Complexity: , where is the length of positions. We use two for-loops, each of complexity .

• Space Complexity: , the space used by qans and ans.

Approach 2: Brute Force with Coordinate Compression

Intuition and Algorithm

Let N = len(positions). After mapping the board to a board of length at most , we can brute force the answer by simulating each square's drop directly.

Our answer is either the current answer or the height of the square that was just dropped, and we'll update it appropriately.

Complexity Analysis

• Time Complexity: , where is the length of positions. We use two for-loops, each of complexity (because of coordinate compression.)

• Space Complexity: , the space used by heights.

Approach 3: Block (Square Root) Decomposition

Intuition

Whenever we perform operations (like update and query) on some interval in a domain, we could segment that domain with size into blocks of size .

Then, instead of a typical brute force where we update our array heights representing the board, we will also hold another array blocks, where blocks[i] represents the elements heights[B*i], heights[B*i + 1], ..., heights[B*i + B-1]. This allows us to write to the array in operations.

Algorithm

Let's get into the details. We actually need another array, blocks_read. When we update some element i in block b = i / B, we'll also update blocks_read[b]. If later we want to read the entire block, we can read from here (and stuff written to the whole block in blocks[b].)

When we write to a block, we'll write in blocks[b]. Later, when we want to read from an element i in block b = i / B, we'll read from heights[i] and blocks[b].

Our process for managing query and update will be similar. While left isn't a multiple of B, we'll proceed with a brute-force-like approach, and similarly for right. At the end, [left, right+1) will represent a series of contiguous blocks: the interval will have length which is a multiple of B, and left will also be a multiple of B.

Complexity Analysis

• Time Complexity: , where is the length of positions. Each query and update has complexity .

• Space Complexity: , the space used by heights.

Approach 4: Segment Tree with Lazy Propagation

Intuition

If we were familiar with the idea of a segment tree (which supports queries and updates on intervals), we can immediately crack the problem.

Algorithm

Segment trees work by breaking intervals into a disjoint sum of component intervals, whose number is at most log(width). The motivation is that when we change an element, we only need to change log(width) many intervals that aggregate on an interval containing that element.

When we want to update an interval all at once, we need to use lazy propagation to ensure good run-time complexity. This topic is covered in more depth here.

With such an implementation in hand, the problem falls out immediately.

Complexity Analysis

• Time Complexity: , where is the length of positions. This is the run-time complexity of using a segment tree.

• Space Complexity: , the space used by our tree.

Analysis written by: @awice.