Problem - Leetcode

Design a time-based key-value data structure that can store multiple values for the same key at different time stamps and retrieve the key's value at a certain timestamp.

Implement the TimeMap class:

• TimeMap() Initializes the object of the data structure.

• void set(String key, String value, int timestamp) Stores the key key with the value value at the given time timestamp.

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

Example 1:

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

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

Constraints:

• 1 <= key.length, value.length <= 100

• key and value consist of lowercase English letters and digits.

• 1 <= timestamp <= 10<sup>7</sup>

• All the timestamps timestamp of set are strictly increasing.

• At most 2 * 10<sup>5</sup> calls will be made to set and get.

Solution in Golang

type TimeMap struct {
    store map[string][]ValStamp
}

type ValStamp struct {
    Val  string
    Time int
}

func Constructor() TimeMap {
    return TimeMap{store: make(map[string][]ValStamp)}
}

func (this *TimeMap) Set(key string, value string, timestamp int) {
    if _, ok := this.store[key]; !ok {
        this.store[key] = make([]ValStamp, 0)
    }
    this.store[key] = append(this.store[key], ValStamp{value, timestamp})
}

func (this *TimeMap) Get(key string, timestamp int) string {
    var res string
    var values []ValStamp
    if _, ok := this.store[key]; ok {
        values = this.store[key]
    }
    l, r := 0, len(values)-1
    for l <= r {
        mid := l + (r-l)/2
        if values[mid].Time <= timestamp {
            res = values[mid].Val
            l = mid + 1
        } else {
            r = mid - 1
        }
    }
    return res
}

This code defines a TimeMap struct, which is essentially a key-value store where each key can have multiple values associated with different timestamps. Here's a detailed breakdown:

1. TimeMap Struct:

   • TimeMap is defined with a single field, store, which is a map with string keys and slices of ValStamp values.

   • ValStamp is a struct representing a value and its associated timestamp.

2. Constructor:

   • The Constructor function initializes and returns a new TimeMap with an empty map as its store.

3. Set Method:

   • Set adds a new value along with its timestamp to the map for a given key.

   • If the key doesn't exist in the map, it initializes an empty slice for that key.

   • It then appends a new ValStamp to the slice with the provided value and timestamp.

4. Get Method:

   • Get retrieves the value associated with a key at or before the specified timestamp.

   • It initializes two pointers, l (left) and r (right), for a binary search within the stored values.

   • The function iteratively performs a binary search to find the closest timestamp that is less than or equal to the given timestamp.

   • The result is updated as it searches, and the final result is the value associated with the found timestamp.

In summary, this code implements a time-based key-value store where values are associated with timestamps, and the Get method retrieves the value closest to or at a specified timestamp using binary search on the stored timestamps.