java: add «1942. The Number of the Smallest Unoccupied Chair»

URL:	https://leetcode.com/problems/the-number-of-the-smallest-unoccupied-chair/
Signed-off-by: Matej Focko <me@mfocko.xyz>

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

@@ -0,0 +1,35 @@
+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;
+  }
+}