Time Based Key-Value Store

Design, Binary Search, TreeMap, HashMap

Medium

Create a timebased key-value store class TimeMap, that supports two operations.

1.set(string key, string value, int timestamp)

  • Stores the key and value, along with the given timestamp.

2.get(string key, int timestamp)

  • Returns a value such that set(key, value, timestamp_prev) was called previously, with timestamp_prev <= timestamp.
  • If there are multiple such values, it returns the one with the largest timestamp_prev.
  • If there are no values, it returns the empty string ("").

Example 1:

Input: 
inputs = 
["TimeMap","set","get","get","set","get","get"]
, inputs = 
[[],["foo","bar",1],["foo",1],["foo",3],["foo","bar2",4],["foo",4],["foo",5]]
Output: 
[null,null,"bar","bar",null,"bar2","bar2"]
Explanation: 

TimeMap kv;   
kv.set("foo", "bar", 1); // store the key "foo" and value "bar" along with timestamp = 1   
kv.get("foo", 1);  // output "bar"   
kv.get("foo", 3); // output "bar" since there is no value corresponding to foo at timestamp 3 and timestamp 2, then the only value is at timestamp 1 ie "bar"   
kv.set("foo", "bar2", 4);   
kv.get("foo", 4); // output "bar2"   
kv.get("foo", 5); //output "bar2"

Example 2:

Input: 
inputs = 
["TimeMap","set","set","get","get","get","get","get"]
, inputs = 
[[],["love","high",10],["love","low",20],["love",5],["love",10],["love",15],["love",20],["love",25]]
Output: 
[null,null,null,"","high","high","low","low"]

Note:

  1. All key/value strings are lowercase.
  2. All key/value strings have length in the range [1, 100]
  3. The timestamps for all TimeMap.set operations are strictly increasing.
  4. 1 <= timestamp <= 10^7
  5. TimeMap.set and TimeMap.get functions will be called a total of 120000 times (combined) per test case.

Analysis & Solution

HashMap + TreeMap

  • Time Complexity: O(1) for each set operation, and O(logN) for eachgetoperation, where N is the number of entries in theTimeMap.

  • Space Complexity: O(N).

class TimeMap {

    HashMap<String, TreeMap<Integer, String>> tm;

    /** Initialize your data structure here. */
    public TimeMap() {
        tm = new HashMap<>();
    }

    public void set(String key, String value, int timestamp) {
        if (!tm.containsKey(key)) {
            tm.put(key, new TreeMap<Integer, String>());
        }
        tm.get(key).put(timestamp, value);
    }

    public String get(String key, int timestamp) {
        if (!tm.containsKey(key)) {
            return "";
        }
        TreeMap<Integer, String> tree = tm.get(key);
        Integer t = tree.floorKey(timestamp);
        if (t == null) {
            return "";
        }
        return tree.get(t);
    }
}

/**
 * Your TimeMap object will be instantiated and called as such:
 * TimeMap obj = new TimeMap();
 * obj.set(key,value,timestamp);
 * String param_2 = obj.get(key,timestamp);
 */

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