Question

C++ Problem

I want to understand why did we use (1001, vector<int>()) when we initialize the data structure?

I want to understand why are we using /1000 and %1000 for the hash1 and hash2 values?

The code is below the question

Question

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

To be specific, your design should include these functions:

• put(key, value) : Insert a (key, value) pair into the HashMap. If the value already exists in the HashMap, update the value.
• get(key): Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key.
• remove(key) : Remove the mapping for the value key if this map contains the mapping for the key.

Example:

MyHashMap hashMap = new MyHashMap();
hashMap.put(1, 1);          
hashMap.put(2, 2);         
hashMap.get(1);            // returns 1
hashMap.get(3);            // returns -1 (not found)
hashMap.put(2, 1);          // update the existing value
hashMap.get(2);            // returns 1 
hashMap.remove(2);          // remove the mapping for 2
hashMap.get(2);            // returns -1 (not found) 

Note:

• All keys and values will be in the range of [0, 1000000].
• The number of operations will be in the range of [1, 10000].
• Please do not use the built-in HashMap library.

Code Answer

class MyHashMap {
public:
    /** Initialize your data structure here. */
    MyHashMap() {
        myMap = vector<vector<int>>(1001,vector<int>());
    }
    
    /** value will always be non-negative. */
    void put(int key, int value) {
        int hash1 = key/1000;
        int hash2 = key%1000;
        if (myMap[hash1].empty()) myMap[hash1].resize(1000,-1);
        myMap[hash1][hash2] = value;
    }
    
    /** Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key */
    int get(int key) {
        int hash1 = key/1000;
        int hash2 = key%1000;
        if (myMap[hash1].empty()) return -1;
        return myMap[hash1][hash2];
    }
    
    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    void remove(int key) {
        int hash1 = key/1000;
        int hash2 = key%1000;
        if (myMap[hash1].empty()) return ;
        myMap[hash1][hash2] = -1;
    }

    vector<vector<int>> myMap;
};

Answer 1

• we used  (1001, vector<int>()) when we initialize the data structure because, in myMap, we are storing 1001 vectors of type int with default size each.
• we using /1000 and %1000 for the hash1 and hash2 values as our each vector side is 1000 max and it can't store more elements then 1000 so using '%' and '/' reduce size of hash to <1000