Leetcode 981. Time Based Key-Value Store

medium

C++ stl container is very convenient for this problem

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 "ba2r" along with timestamp = 4.
timeMap.get("foo", 4);         // return "bar2"
timeMap.get("foo", 5);         // return "bar2"

Solution

1. Don’t use unordered_multimap.

• we should not keep equivalent keys
• multimap does’t support [] operation. So inconvenient!

2. for key use unordered_map, but for timestamp use map

• key needs to be associative
• timestamp needs to be ordered for a time-bound search

class TimeMap {  
    unordered_map<string, map<int, string>> hash; 
    // key : [ timestamp: value]
    
public:
    /** Initialize your data structure here. */
    TimeMap() {
        
    }
    
    void set(string key, string value, int timestamp) {
        hash[key][timestamp] = value;       
    }
    
    string get(string key, int timestamp) {
        if (hash.find(key) != hash.end()) {
            map<int, string>& m = hash.find(key)->second;
            map<int, string>::iterator itr 
                = m.upper_bound(timestamp);
            if (itr == m.begin()) {   
                return "";
            } else {
                return prev(itr)->second;
            }
        }
        return "";
    }
};

lower_bound: first equal to (or greater than when not found).

upper_bound: first greater than