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Problem solution · Java

Time Based Key-Value Store

Time Based Key-Value Store: a Java solution using sliding window or two pointers. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Sliding window or two pointers
Source
walkccc LeetCode Solutions
Length
37 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

Sliding window or two pointers

For Time Based Key-Value Store, the implementation maintains a moving interval and updates only the information that enters or leaves the window.

  1. Choose the invariant that makes a window valid or useful.
  2. Advance the right boundary and add the new element.
  3. Move the left boundary only as needed while maintaining the invariant and updating the answer.

Code notes

  • 37 lines of Java from the credited upstream file 981.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • 1 loop block detected.

Complexity

Confirm that neither pointer moves backwards; if so, the scan is usually linear apart from the window’s data-structure operations.

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 codeTime Based Key-Value Store · JavaJava
Use this to learn the idea, then write your own version.
class T {  public String value;  public int timestamp;  public T(String value, int timestamp) {    this.value = value;    this.timestamp = timestamp;  }} class TimeMap {  public void set(String key, String value, int timestamp) {    map.putIfAbsent(key, new ArrayList<>());    map.get(key).add(new T(value, timestamp));  }   public String get(String key, int timestamp) {    List<T> A = map.get(key);    if (A == null)      return "";     int l = 0;    int r = A.size();     while (l < r) {      final int m = (l + r) / 2;      if (A.get(m).timestamp > timestamp)        r = m;      else        l = m + 1;    }     return l == 0 ? "" : A.get(l - 1).value;  }   private Map<String, List<T>> map = new HashMap<>();}

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