2013. Detect Squares

Problem Link

Description

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You are given a stream of points on the X-Y plane. Design an algorithm that:

• Adds new points from the stream into a data structure. Duplicate points are allowed and should be treated as different points.
• Given a query point, counts the number of ways to choose three points from the data structure such that the three points and the query point form an axis-aligned square with positive area.

An axis-aligned square is a square whose edges are all the same length and are either parallel or perpendicular to the x-axis and y-axis.

Implement the DetectSquares class:

• DetectSquares() Initializes the object with an empty data structure.
• void add(int[] point) Adds a new point point = [x, y] to the data structure.
• int count(int[] point) Counts the number of ways to form axis-aligned squares with point point = [x, y] as described above.

 

Example 1:

Input
["DetectSquares", "add", "add", "add", "count", "count", "add", "count"]
[[], [[3, 10]], [[11, 2]], [[3, 2]], [[11, 10]], [[14, 8]], [[11, 2]], [[11, 10]]]
Output
[null, null, null, null, 1, 0, null, 2]
Explanation
DetectSquares detectSquares = new DetectSquares();
detectSquares.add([3, 10]);
detectSquares.add([11, 2]);
detectSquares.add([3, 2]);
detectSquares.count([11, 10]); // return 1. You can choose:
//   - The first, second, and third points
detectSquares.count([14, 8]);  // return 0. The query point cannot form a square with any points in the data structure.
detectSquares.add([11, 2]);    // Adding duplicate points is allowed.
detectSquares.count([11, 10]); // return 2. You can choose:
//   - The first, second, and third points
//   - The first, third, and fourth points

 

Constraints:

• point.length == 2
• 0 <= x, y <= 1000
• At most 3000 calls in total will be made to add and count.

Solution

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Python3

class DetectSquares:
 
    def __init__(self):
        self.xmap = collections.defaultdict(set)
        self.mp = collections.defaultdict(int)
 
    def add(self, point: List[int]) -> None:
        self.mp[tuple(point)] += 1
        self.xmap[point[0]].add(point[1])
 
    def count(self, point: List[int]) -> int:
        x1, y1 = point
        res = 0
        
        for y2 in self.xmap[x1]:
            d = abs(y2 - y1)
            if d == 0: continue
            for x3, y3 in [(x1 + d, y1), (x1 - d, y1)]:
                x4, y4 = x3, y2
                
                res += self.mp[(x1, y2)] * self.mp[(x3, y3)] * self.mp[(x4, y4)]
        
        return res
 
 
# Your DetectSquares object will be instantiated and called as such:
# obj = DetectSquares()
# obj.add(point)
# param_2 = obj.count(point)