CPMagical CP Library

Problem solution · Java

The Number of the Smallest Unoccupied Chair

The Number of the Smallest Unoccupied Chair: a Java solution using heap or priority queue. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Heap or priority queue
Source
walkccc LeetCode Solutions
Length
34 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Heap or priority queue

For The Number of the Smallest Unoccupied Chair, the implementation repeatedly takes the currently best candidate from a heap while inserting newly available choices.

  1. Define the priority key and whether the smallest or largest item should lead.
  2. Push each candidate when it becomes eligible.
  3. Discard stale entries when necessary and process the best live candidate.

Code notes

  • 34 lines of Java from the credited upstream file 1942.java.
  • The implementation visibly relies on sequence storage, work queue.
  • 3 loop blocks detected.

Complexity

Count heap pushes and pops; each normally contributes a logarithmic factor in the heap size.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeThe Number of the Smallest Unoccupied Chair · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public int smallestChair(int[][] times, int targetFriend) {    int nextUnsatChair = 0;    PriorityQueue<Integer> emptyChairs = new PriorityQueue<>();    PriorityQueue<Pair<Integer, Integer>> occupied =        new PriorityQueue<>(Comparator.comparingInt(Pair::getKey));     for (int i = 0; i < times.length; ++i) {      int[] time = times[i];      time = Arrays.copyOf(time, time.length + 1);      time[time.length - 1] = i;      times[i] = time;    }     Arrays.sort(times, Comparator.comparingInt(time -> time[0]));     for (int[] time : times) {      final int arrival = time[0];      final int leaving = time[1];      final int i = time[2];      while (!occupied.isEmpty() && occupied.peek().getKey() <= arrival)        emptyChairs.add(occupied.poll().getValue());      if (i == targetFriend)        return emptyChairs.isEmpty() ? nextUnsatChair : emptyChairs.peek();      if (emptyChairs.isEmpty())        occupied.add(new Pair<>(leaving, nextUnsatChair++));      else        occupied.add(new Pair<>(leaving, emptyChairs.poll()));    }     throw new IllegalArgumentException();  }}

Keep practising

Did this explanation save you time? I'm a Grade 11 student building this free library to make difficult algorithms easier to understand.

Buy me a coffee ↗