Same feeling, not easy. Need to heaps approach ``` class Solution { private class Rental { int vehicleNum; long start; long end; public Rental(long start, long end, int vehicleNum){ this.vehicleNum = vehicleNum; this.start = start; this.end = end; } } public List> assignRentalFleet(List> reservations) { List> res = new ArrayList<>(); // sort reservations Collections.sort(reservations, (a, b) -> { long sa = Long.parseLong(a.get(1)); long sb = Long.parseLong(b.get(1)); long ea = Long.parseLong(a.get(2)); long eb = Long.parseLong(b.get(2)); String ida = a.get(0); String idb = b.get(0); // 1. 先比start,在比end if (sa != sb){ return Long.compare(sa, sb); } // 2. 在比end time if (ea != eb){ return Long.compare(ea, eb); } // 3.最後比ascii id return ida.compareTo(idb); }); PriorityQueue busy = new PriorityQueue<>( (a, b) -> { if (a.end == b.end) return Integer.compare(a.vehicleNum, b.vehicleNum); return Long.compare(a.end, b.end); }); // availabel vehicle sort by smallest car number PriorityQueue available = new PriorityQueue<>(); int currentNum = 0; for (List record : reservations){ String id = record.get(0); long start = Long.parseLong(record.get(1)); long end = Long.parseLong(record.get(2)); int num; // relese return cars while (!busy.isEmpty() && busy.peek().end <= start){ Rental r = busy.poll(); available.offer(r.vehicleNum); } // find spot in available queue if (!available.isEmpty()){ num = available.poll(); } else { currentNum++; num = currentNum; } // add into busy queue and add into result res.add(List.of("assignment", id, String.valueOf(num))); busy.offer(new Rental(start, end, num)); } // sort output result by id Collections.sort(res, (a, b) -> a.get(1).compareTo(b.get(1))); res.addFirst(List.of("fleet", String.valueOf(currentNum))); return res; } } ```

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