Check if Grid can be Cut into Sections Interview Question for Google

You are given an integer n representing the dimensions of an n x n grid, with the origin at the bottom-left corner of the grid. You are also given a 2D array of coordinates rectangles, where rectangles[i] is in the form [startx, starty, endx, endy], representing a rectangle on the grid. Each rectangle is defined as follows:

(startx, starty): The bottom-left corner of the rectangle.
(endx, endy): The top-right corner of the rectangle.

Note that the rectangles do not overlap. Your task is to determine if it is possible to make either two horizontal or two vertical cuts on the grid such that:

Each of the three resulting sections formed by the cuts contains at least one rectangle.
Every rectangle belongs to exactly one section.

Return true if such cuts can be made; otherwise, return false.

For example:

n = 5, rectangles = [[1,0,5,2],[0,2,2,4],[3,2,5,3],[0,4,4,5]] should return true because we can make horizontal cuts at y = 2 and y = 4.

n = 4, rectangles = [[0,0,1,1],[2,0,3,4],[0,2,2,3],[3,0,4,3]] should return true because we can make vertical cuts at x = 2 and x = 3.

n = 4, rectangles = [[0,2,2,4],[1,0,3,2],[2,2,3,4],[3,0,4,2],[3,2,4,4]] should return false because we cannot make two horizontal or two vertical cuts that satisfy the conditions.

What approach would you take to solve this problem? What is the time and space complexity of your solution?

Naive Approach

A brute force approach would involve iterating through all possible pairs of horizontal and vertical cuts and checking if the resulting sections satisfy the given conditions. We can generate all possible pairs of cuts and then iterate through the rectangles to check which section they belong to. This would have a high time complexity.

Optimal Solution

We can optimize the solution by first trying horizontal cuts and then vertical cuts. For each direction, we iterate through all possible pairs of cuts. For a given pair of cuts, we check if each of the three sections contains at least one rectangle and that every rectangle belongs to exactly one section.

Check Horizontal Cuts:
- Iterate through all possible y values for the first cut (from 1 to n-2).
- Iterate through all possible y values for the second cut (from first cut + 1 to n-1).
- For each pair of horizontal cuts, check if the conditions are met.
Check Vertical Cuts:
- Iterate through all possible x values for the first cut (from 1 to n-2).
- Iterate through all possible x values for the second cut (from first cut + 1 to n-1).
- For each pair of vertical cuts, check if the conditions are met.

Here's a Python implementation of the optimal solution:

def solve():
    n = 5
    rectangles = [[1, 0, 5, 2], [0, 2, 2, 4], [3, 2, 5, 3], [0, 4, 4, 5]]

    def check_cuts(cuts, horizontal):
        sections = [[] for _ in range(3)]
        for rect in rectangles:
            start_x, start_y, end_x, end_y = rect
            if horizontal:
                if start_y < cuts[0]:
                    sections[0].append(rect)
                elif start_y < cuts[1]:
                    sections[1].append(rect)
                else:
                    sections[2].append(rect)
            else:
                if start_x < cuts[0]:
                    sections[0].append(rect)
                elif start_x < cuts[1]:
                    sections[1].append(rect)
                else:
                    sections[2].append(rect)

        for section in sections:
            if not section:
                return False

        all_rects = []
        for section in sections:
            all_rects.extend(section)

        if len(all_rects) != len(rectangles):
            return False

        return True

    def check_horizontal_cuts():
        for i in range(1, n - 1):
            for j in range(i + 1, n):
                if check_cuts([i, j], True):
                    return True
        return False

    def check_vertical_cuts():
        for i in range(1, n - 1):
            for j in range(i + 1, n):
                if check_cuts([i, j], False):
                    return True
        return False

    return check_horizontal_cuts() or check_vertical_cuts()

print(solve())

Time and Space Complexity

Time Complexity: O(n^2 * m), where n is the dimension of the grid and m is the number of rectangles. The n^2 comes from iterating through pairs of cuts and m is iterating through the rectangles to check if they satisfy the condition. In the worst case, the loops go through all possible n values, and the check cuts function goes through each rectangle, therefore the time complexity is O(n^2 * m).
Space Complexity: O(m). The space complexity is O(1) because the algorithm uses a constant amount of extra space, excluding the input. However, if we include the space taken by the section lists in the check_cuts function, the space complexity is O(m) because in the worst case, all rectangles will be added to these lists.

Edge Cases

Insufficient Rectangles: If there are fewer than three rectangles, it's impossible to satisfy the conditions.
Rectangles Concentrated in One Area: If all rectangles are concentrated in a small area, it might not be possible to make cuts that separate them properly.
Invalid Input: If the input rectangles have invalid coordinates (e.g., start_x >= end_x or start_y >= end_y), the algorithm should handle them gracefully (e.g., by returning false or raising an exception).