notes/OJ notes/pages/Leetcode Design-Hash.md

Leetcode Design-Hash

2022-09-01 14:44

Data structures:

#DS #array #linked_list

Difficulty:

#coding_problem #difficulty_easy

Additional tags:

#leetcode #CS_list_need_practicing

Revisions:

N/A

Links:

• Link to problem
• Explanation

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Problem

Design a HashMap without using any built-in hash table libraries.

Implement the MyHashMap class:

• MyHashMap() initializes the object with an empty map.
• void put(int key, int value) inserts a (key, value) pair into the HashMap. If the key already exists in the map, update the corresponding value.
• int get(int key) returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key.
• void remove(key) removes the key and its corresponding value if the map contains the mapping for the key.

Examples

Example 1:

Input ["MyHashMap", "put", "put", "get", "get", "put", "get", "remove", "get"] ], [1, 1], [2, 2], [1], [3], [2, 1], [2], [2], [2 Output [null, null, null, 1, -1, null, 1, null, -1]

Explanation MyHashMap myHashMap = new MyHashMap(); myHashMap.put(1, 1); // The map is now 1,1 myHashMap.put(2, 2); // The map is now 1,1], [2,2 myHashMap.get(1); // return 1, The map is now 1,1], [2,2 myHashMap.get(3); // return -1 (i.e., not found), The map is now 1,1], [2,2 myHashMap.put(2, 1); // The map is now 1,1], [2,1 (i.e., update the existing value) myHashMap.get(2); // return 1, The map is now 1,1], [2,1 myHashMap.remove(2); // remove the mapping for 2, The map is now 1,1 myHashMap.get(2); // return -1 (i.e., not found), The map is now 1,1

Constraints

• 0 <= key, value <= 106
• At most 104 calls will be made to put, get, and remove.

Thoughts

[!summary] This is a #array problem.

Easy way:

Using array, since 10^6 size is not too large

Hard way:

Using hashes with linked list to support collision check.

Solution

Easy way:

class MyHashMap {
  int myMap[1000001];

public:
  MyHashMap() { fill(myMap, myMap + 1000001, -1); }

  void put(int key, int value) { myMap[key] = value; }

  int get(int key) { return myMap[key]; }

  void remove(int key) { myMap[key] = -1; }
};

/**
 * Your MyHashMap object will be instantiated and called as such:
 * MyHashMap* obj = new MyHashMap();
 * obj->put(key,value);
 * int param_2 = obj->get(key);
 * obj->remove(key);
 */