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.

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 <= 10^6
At most 10^4 calls will be made to put, get, and remove.

Follow up: Please do not use the built-in HashMap library.

Solution

class HTListNode {
public:
    int key, val;
    HTListNode *next;
    HTListNode(int key_, int val_): key(key_), val(val_), next(nullptr) {}
    ~HTListNode() {}
};

class MyHashMap {
public:
    /** Initialize your data structure here. */
    MyHashMap() {
        for (int i=0; i<1000; ++i)
            m_[i] = new HTListNode(-1, -1);
    }
    
    /** value will always be non-negative. */
    void put(int key, int value) {
        int idx = mod_f(key);
        HTListNode *prev = m_[idx];
        HTListNode *cur = prev->next;
        while (cur && cur->key != key) {
            prev = cur;
            cur = cur->next;
        }
        
        if (cur != nullptr) {
            cur->val = value;
        } else {
            prev->next = new HTListNode(key, 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 idx = mod_f(key);
        HTListNode *cur = m_[idx];
        while (cur && cur->key != key) cur = cur->next;
        if (cur != nullptr) return cur->val;
        else return -1;
    }
    
    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    void remove(int key) {
        int idx = mod_f(key);
        HTListNode *prev = m_[idx];
        HTListNode *cur = prev->next;
        while (cur && cur->key != key) {
            prev = cur;
            cur = cur->next;
        }
        
        if (cur != nullptr) {
            prev->next = cur->next;
            delete cur;
        }
    }
    
    int mod_f(int key) {
        return key % 1000;
    }
private:
    HTListNode *m_[1000];
};

/**
 * 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);
 */

概念： key -> hashing function -> idx -> linked list.

此處 hashing function 用 mode。
範圍鎖定在給的容器大小，這邊是 1000。
得到 hashing 後的 idx 後，若有 collision，則用 linked list 串起來。

初始化時的 linked list 是 dummy head。
put, get, remove，就是 linked list 的新增、尋訪和刪除。