g0801_0900.s0855_exam_room.ExamRoom Maven / Gradle / Ivy

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package g0801_0900.s0855_exam_room;

// #Medium #Design #Ordered_Set #2022_03_27_Time_19_ms_(74.01%)_Space_45.8_MB_(67.28%)

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;

public class ExamRoom {
    private static class Node {
        Node pre;
        Node next;
        int val;

        Node(int val, Map map) {
            this.val = val;
            map.put(val, this);
        }

        int insert(Node left) {
            Node right = left.next;
            left.next = this;
            right.pre = this;
            this.next = right;
            this.pre = left;
            return val;
        }

        void delete() {
            Node left = this.pre;
            Node right = this.next;
            left.next = right;
            right.pre = left;
        }
    }

    private Map map = new HashMap<>();
    private Node head = new Node(-1, map);
    private Node tail = new Node(-1, map);
    private int n;

    public ExamRoom() {
        head.next = tail;
        tail.pre = head;
    }

    public ExamRoom(int n) {
        this();
        this.n = n;
    }

    public int seat() {
        int right = n - 1 - tail.pre.val;
        int left = head.next.val;
        int max = 0;
        int maxAt = -1;
        Node maxAtLeft = null;
        for (Node cur = tail.pre; cur != head && cur.pre != head; cur = cur.pre) {
            Node pre = cur.pre;
            int at = (cur.val + pre.val) / 2;
            int distance = at - pre.val;
            if (distance >= max) {
                maxAtLeft = pre;
                max = distance;
                maxAt = at;
            }
        }
        if (head.next == tail || left >= max && left >= right) {
            return new Node(0, map).insert(head);
        }
        if (right > max) {
            return new Node(n - 1, map).insert(tail.pre);
        }
        return new Node(maxAt, map).insert(Objects.requireNonNull(maxAtLeft));
    }

    public void leave(int p) {
        map.get(p).delete();
        map.remove(p);
    }
}

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