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Problem solution · Java

Design Underground System

Design Underground System: a Java solution using hash-based lookup. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Hash-based lookup
Source
walkccc LeetCode Solutions
Length
41 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

Hash-based lookup

For Design Underground System, the implementation stores previously seen values or frequencies in a hash table for direct membership and lookup operations.

  1. Decide the key that represents the information needed later.
  2. Update its count or stored state while scanning the input.
  3. Use constant-time expected lookups to detect matches or assemble the result.

Code notes

  • 41 lines of Java from the credited upstream file 1396.java.
  • The implementation visibly relies on hash lookup, ordered lookup.
  • No explicit loop blocks detected.

Complexity

Expected hash operations are constant time, but the surrounding scan and the number of stored keys determine total work and memory.

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 codeDesign Underground System · JavaJava
Use this to learn the idea, then write your own version.
class CheckIn {  public String stationName;  public int time;  public CheckIn(String stationName, int time) {    this.stationName = stationName;    this.time = time;  }} class CheckOut {  public int numTrips;  public int totalTime;  public CheckOut(int numTrips, int totalTime) {    this.numTrips = numTrips;    this.totalTime = totalTime;  }} class UndergroundSystem {  public void checkIn(int id, String stationName, int t) {    checkIns.put(id, new CheckIn(stationName, t));  }   public void checkOut(int id, String stationName, int t) {    final CheckIn checkIn = checkIns.get(id);    checkIns.remove(id);    final String route = checkIn.stationName + "->" + stationName;    checkOuts.putIfAbsent(route, new CheckOut(0, 0));    ++checkOuts.get(route).numTrips;    checkOuts.get(route).totalTime += t - checkIn.time;  }   public double getAverageTime(String startStation, String endStation) {    final CheckOut checkOut = checkOuts.get(startStation + "->" + endStation);    return checkOut.totalTime / (double) checkOut.numTrips;  }   private Map<Integer, CheckIn> checkIns = new HashMap<>();  // {id: (stationName, time)}  private Map<String, CheckOut> checkOuts = new HashMap<>(); // {route: (numTrips, totalTime)}}

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