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admin 11月 13, 2020 0

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.

Build a hashmap by linkedlist.

  • An array of hashNodes represents an array of slots. And the number of slots is n.
  • hash fuction is simple: hash_index = key % n.
  • every slot which is the first node stored in the array is a dummy node(key = -1, value = -1).
  • If we put a <key, value>, we need to
    • check if the slot exists
    • check if share the slot with other nodes having different keys.
    • check if the key exists. 
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struct HashNode {
    int value;
    int key;
    HashNode* next;
    HashNode(int k, int v): value(v), key(k), next(NULL){}
};
class  {
    vector<HashNode*> list;
public:
    
    MyHashMap() {
        list = vector<HashNode*>(10000, NULL);
    }

    /** value will always be non-negative. */
    void put(int key, int value) {
        int ind = hashFunc(key);
        if(list[ind] == NULL)
            list[ind] = new HashNode(-1, -1);
         HashNode* pre = findPre(list[ind], key);
        if(pre->next == NULL){
            pre->next = new HashNode(key, value);
        }
        else{
            pre->next->value = 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 ind = hashFunc(key);
        if(list[ind] == NULL)
            return -1;
        HashNode* pre = findPre(list[ind], key);
        return pre->next == NULL ? -1 : pre->next->value;
    }

    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    void remove(int key) {
        int ind = hashFunc(key);
        if(!list[ind])
            return;
        HashNode* pre = findPre(list[ind], key);
       if(pre->next == NULL)
           return;
        pre->next = pre->next->next;
    }

    int hashFunc(int key){
        return key % 10000;
    }
    HashNode* findPre(HashNode* slot, int key){
        HashNode* pre = NULL;
        HashNode* cur = slot;
        while(cur != NULL && cur->key != key){
            pre = cur;
            cur = cur->next;
        }
        return pre;
    }
};
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