3531 - Count Covered Buildings.

Medium

You are given a positive integer n, representing an n x n city. You are also given a 2D grid buildings, where buildings[i] = [x, y] denotes a unique building located at coordinates [x, y].

A building is covered if there is at least one building in all four directions: left, right, above, and below.

Return the number of covered buildings.

Example 1:

Input: n = 3, buildings = [1,2,2,2,3,2,2,1,2,3]

Output: 1

Explanation:

• Only building [2,2] is covered as it has at least one building:

• Thus, the count of covered buildings is 1.

Example 2:

Input: n = 3, buildings = [1,1,1,2,2,1,2,2]

Output: 0

Explanation:

• No building has at least one building in all four directions.

Example 3:

Input: n = 5, buildings = [1,3,3,2,3,3,3,5,5,3]

Output: 1

Explanation:

• Only building [3,3] is covered as it has at least one building:

• Thus, the count of covered buildings is 1.

Constraints:

• <code>2 <= n <= 10⁵</code>

• <code>1 <= buildings.length <= 10⁵</code>

• buildings[i] = [x, y]

• 1 <= x, y <= n

• All coordinates of buildings are unique.