Question

3025. Find the Number of Ways to Place People I

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Description

You are given a 2D array points of size n x 2 representing integer coordinates of some points on a 2D-plane, where points[i] = [xi, yi] .

We define the right direction as positive x-axis (increasing x-coordinate) and the left direction as negative x-axis (decreasing x-coordinate). Similarly, we define the up direction as positive y-axis (increasing y-coordinate) and the down direction as negative y-axis (decreasing y-coordinate)

You have to place n people, including Alice and Bob, at these points such that there is exactly one person at every point. Alice wants to be alone with Bob, so Alice will build a rectangular fence with Alice's position as the upper left corner and Bob's position as the lower right corner of the fence (Note that the fence might not enclose any area, i.e. it can be a line). If any person other than Alice and Bob is either inside the fence or on the fence, Alice will be sad.

Return _the number of pairs of points where you can place Alice and Bob, such that Alice does not become sad on building the fence_.

Note that Alice can only build a fence with Alice's position as the upper left corner, and Bob's position as the lower right corner. For example, Alice cannot build either of the fences in the picture below with four corners (1, 1) , (1, 3) , (3, 1) , and (3, 3) , because:

• With Alice at (3, 3) and Bob at (1, 1) , Alice's position is not the upper left corner and Bob's position is not the lower right corner of the fence.
• With Alice at (1, 3) and Bob at (1, 1) , Bob's position is not the lower right corner of the fence.

Example 1:

Input: points = [[1,1],[2,2],[3,3]]
Output: 0
Explanation: There is no way to place Alice and Bob such that Alice can build a fence with Alice's position as the upper left corner and Bob's position as the lower right corner. Hence we return 0. 

Example 2:

Input: points = [[6,2],[4,4],[2,6]]
Output: 2
Explanation: There are two ways to place Alice and Bob such that Alice will not be sad:
- Place Alice at (4, 4) and Bob at (6, 2).
- Place Alice at (2, 6) and Bob at (4, 4).
You cannot place Alice at (2, 6) and Bob at (6, 2) because the person at (4, 4) will be inside the fence.

Example 3:

Input: points = [[3,1],[1,3],[1,1]]
Output: 2
Explanation: There are two ways to place Alice and Bob such that Alice will not be sad:
- Place Alice at (1, 1) and Bob at (3, 1).
- Place Alice at (1, 3) and Bob at (1, 1).
You cannot place Alice at (1, 3) and Bob at (3, 1) because the person at (1, 1) will be on the fence.
Note that it does not matter if the fence encloses any area, the first and second fences in the image are valid.

Constraints:

• 2 <= n <= 50
• points[i].length == 2
• 0 <= points[i][0], points[i][1] <= 50
• All points[i] are distinct.

Solutions

Solution 1: Sorting and Classification

First, we sort the array. Then, we can classify the results based on the properties of a triangle.

• If the sum of the two smaller numbers is less than or equal to the largest number, it cannot form a triangle. Return "Invalid".
• If the three numbers are equal, it is an equilateral triangle. Return "Equilateral".
• If two numbers are equal, it is an isosceles triangle. Return "Isosceles".
• If none of the above conditions are met, it is a scalene triangle. Return "Scalene".

The time complexity is $O(1)$, and the space complexity is $O(1)$.

class Solution:
  def numberOfPairs(self, points: List[List[int]]) -> int:
    points.sort(key=lambda x: (x[0], -x[1]))
    ans = 0
    for i, (_, y1) in enumerate(points):
      max_y = -inf
      for _, y2 in points[i + 1 :]:
        if max_y < y2 <= y1:
          max_y = y2
          ans += 1
    return ans

class Solution {
  public int numberOfPairs(int[][] points) {
    Arrays.sort(points, (a, b) -> a[0] == b[0] ? b[1] - a[1] : a[0] - b[0]);
    int ans = 0;
    int n = points.length;
    final int inf = 1 << 30;
    for (int i = 0; i < n; ++i) {
      int y1 = points[i][1];
      int maxY = -inf;
      for (int j = i + 1; j < n; ++j) {
        int y2 = points[j][1];
        if (maxY < y2 && y2 <= y1) {
          maxY = y2;
          ++ans;
        }
      }
    }
    return ans;
  }
}

class Solution {
public:
  int numberOfPairs(vector<vector<int>>& points) {
    sort(points.begin(), points.end(), [](const vector<int>& a, const vector<int>& b) {
      return a[0] < b[0] || (a[0] == b[0] && b[1] < a[1]);
    });
    int n = points.size();
    int ans = 0;
    for (int i = 0; i < n; ++i) {
      int y1 = points[i][1];
      int maxY = INT_MIN;
      for (int j = i + 1; j < n; ++j) {
        int y2 = points[j][1];
        if (maxY < y2 && y2 <= y1) {
          maxY = y2;
          ++ans;
        }
      }
    }
    return ans;
  }
};

func numberOfPairs(points [][]int) (ans int) {
  sort.Slice(points, func(i, j int) bool {
    return points[i][0] < points[j][0] || points[i][0] == points[j][0] && points[j][1] < points[i][1]
  })
  for i, p1 := range points {
    y1 := p1[1]
    maxY := math.MinInt32
    for _, p2 := range points[i+1:] {
      y2 := p2[1]
      if maxY < y2 && y2 <= y1 {
        maxY = y2
        ans++
      }
    }
  }
  return
}

function numberOfPairs(points: number[][]): number {
  points.sort((a, b) => (a[0] === b[0] ? b[1] - a[1] : a[0] - b[0]));
  const n = points.length;
  let ans = 0;
  for (let i = 0; i < n; ++i) {
    const [_, y1] = points[i];
    let maxY = -Infinity;
    for (let j = i + 1; j < n; ++j) {
      const [_, y2] = points[j];
      if (maxY < y2 && y2 <= y1) {
        maxY = y2;
        ++ans;
      }
    }
  }
  return ans;
}

public class Solution {
  public int NumberOfPairs(int[][] points) {
    Array.Sort(points, (a, b) => a[0] == b[0] ? b[1] - a[1] : a[0] - b[0]);
    int ans = 0;
    int n = points.Length;
    int inf = 1 << 30;
    for (int i = 0; i < n; ++i) {
      int y1 = points[i][1];
      int maxY = -inf;
      for (int j = i + 1; j < n; ++j) {
        int y2 = points[j][1];
        if (maxY < y2 && y2 <= y1) {
          maxY = y2;
          ++ans;
        }
      }
    }
    return ans;
  }
}