[LeetCode]Exam Room

题目描述:

LeetCode 855. Exam Room

In an exam room, there are N seats in a single row, numbered 0, 1, 2, ..., N-1.

When a student enters the room, they must sit in the seat that maximizes the distance to the closest person.  If there are multiple such seats, they sit in the seat with the lowest number.  (Also, if no one is in the room, then the student sits at seat number 0.)

Return a class ExamRoom(int N) that exposes two functions: ExamRoom.seat() returning an int representing what seat the student sat in, and ExamRoom.leave(int p) representing that the student in seat number p now leaves the room.  It is guaranteed that any calls to ExamRoom.leave(p) have a student sitting in seat p.

Example 1:

Input: ["ExamRoom","seat","seat","seat","seat","leave","seat"], [[10],[],[],[],[],[4],[]]
Output: [null,0,9,4,2,null,5]
Explanation:
ExamRoom(10) -> null
seat() -> 0, no one is in the room, then the student sits at seat number 0.
seat() -> 9, the student sits at the last seat number 9.
seat() -> 4, the student sits at the last seat number 4.
seat() -> 2, the student sits at the last seat number 2.
leave(4) -> null
seat() -> 5, the student​​​​​​​ sits at the last seat number 5.

Note:

  1. 1 <= N <= 10^9
  2. ExamRoom.seat() and ExamRoom.leave() will be called at most 10^4 times across all test cases.
  3. Calls to ExamRoom.leave(p) are guaranteed to have a student currently sitting in seat number p.

题目大意:

房间里有一排N个座位,编号为0, 1, 2, ..., N-1。

学生进入房间时选择距离最近的人最远的位置就坐。当存在多个满足条件的座位时,选择标号最小的。

学生也可以离开座位。

求每一位学生就坐时的位置。

解题思路:

TreeSet

将每两个座位之间的位置视为“区间”

利用一个TreeSet维护这样的区间,记为pq

用另一个TreeSet维护当前被占用的座位标号,记为seats。

对于leave操作,seats可以用O(log n)的代价找到某座位相邻的座位。并将两个区间合二为一。

对于seat操作,可以通过pq获取当前的最大区间,将区间一分为二。

Java代码:

import java.awt.Point;
import java.util.TreeSet;

class ExamRoom {

    private int N;
    private TreeSet<Integer> seats = new TreeSet<>();
    private TreeSet<Point> pq = new TreeSet<>(
            (Point p1, Point p2) -> {
                int d1 = getDistance(p1);
                int d2 = getDistance(p2);
                if (d1 == d2) {
                    return p1.x - p2.x; 
                }
                return d2 - d1;
            });

    public ExamRoom(int N) {
        this.N = N;
        seats.add(-1);
        seats.add(N);
        pq.add(new Point(-1, N));
    }
    
    private int getDistance(Point p) {
        int mid = getMid(p.x, p.y);
        if (p.x < 0) {
            return p.y - mid;
        } else if (p.y == this.N) {
            return mid - p.x;            
        }
        return Math.min(p.y - mid, mid - p.x);
    }
    
    private int getMid(int left, int right) {
        if (left < 0) return 0;
        if (right == this.N) return this.N - 1;
        return (left + right) / 2;
    }
    
    public int seat() {
        Point p = pq.pollFirst();
        int left = p.x, right = p.y;
        int mid = getMid(left, right);
        seats.add(mid);
        pq.add(new Point(left, mid));
        pq.add(new Point(mid, right));
        return mid;
    }
    
    public void leave(int p) {
        int left = seats.lower(p);
        int right = seats.higher(p);
        seats.remove(p);
        pq.remove(new Point(left, p));
        pq.remove(new Point(p, right));
        pq.add(new Point(left, right));
    }
}

/**
 * Your ExamRoom object will be instantiated and called as such:
 * ExamRoom obj = new ExamRoom(N);
 * int param_1 = obj.seat();
 * obj.leave(p);
 */

 

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