LeetCode/java/the-number-of-the-smallest-unoccupied-chair.java

class Solution {
  public int smallestChair(int[][] times, int targetFriend) {
    int arrival = times[targetFriend][0];
    Arrays.sort(times, (a, b) -> a[0] - b[0]);

    int next = 0;
    var leaving = new PriorityQueue<int[]>((a, b) -> a[0] - b[0]);
    var available = new TreeSet<>();

    for (var range : times) {
      int arrived = range[0], left = range[1];

      while (!leaving.isEmpty() && leaving.peek()[0] <= arrived) {
        available.add(leaving.poll()[1]);
      }

      int current;
      if (!available.isEmpty()) {
        current = (int) available.first();
        available.remove(current);
      } else {
        current = next++;
      }

      if (arrived == arrival) {
        return current;
      }

      leaving.offer(new int[] {left, current});
    }

    // unreachable
    return -1;
  }
}