https://leetcode-cn.com/problems/design-hashset/

思路一：朴实无华且枯燥的链地址法，通过$vector、forward\_list$实现(后者是一个单向链表)。

class MyHashSet {
    
    
public:
    /** Initialize your data structure here. */
    const int num=2333;
    vector<forward_list<int>> hashTable;
    MyHashSet() {
    
    
        hashTable.resize(num);
    }
    
    void add(int key) {
    
    
        if(!contains(key))
            hashTable[key%num].push_front(key);
    }
    
    void remove(int key) {
    
    
        int idx=key%num;
        forward_list<int>::iterator it=hashTable[idx].before_begin();
        forward_list<int>::iterator nxt=hashTable[idx].begin();
        forward_list<int>::iterator ed=hashTable[idx].end();
        while(nxt!=ed&&*nxt!=key)
            it=nxt++;
        if(nxt!=ed)
            hashTable[idx].erase_after(it);
    }
    
    /** Returns true if this set contains the specified element */
    bool contains(int key) {
    
    
        int idx=key%num;
        forward_list<int>::iterator it=hashTable[idx].begin();
        forward_list<int>::iterator ed=hashTable[idx].end();
        while(it!=ed&&*it!=key)
            ++it;
        return it!=ed;
    }
};

/**
 * Your MyHashSet object will be instantiated and called as such:
 * MyHashSet* obj = new MyHashSet();
 * obj->add(key);
 * obj->remove(key);
 * bool param_3 = obj->contains(key);
 */

思路二：位运算实现哈希，这个思想相当巧妙。考虑用一个$unsignedlonglong$数组实现哈希，不妨设模数为$base$，那么对于任意$key$，我们可以把它转换为两部分：$\lfloor key/base\rfloor$和$key\%base$，第一部分决定它在数组中的下标，第二部分决定它在该处值的二进制表示的第几位(这里看代码应该更加直观)。考虑到$ull$类型的取值范围为$[0,2^{64}-1]$，一个$ull$类型最多对应64个数值，所以应取$base=64$，数组的范围应该为$\lfloor max\_key/base\rfloor +1$。

class MyHashSet {
    
    
public:
    /** Initialize your data structure here. */
    const int base=64;
    vector<unsigned long long> hashTable;
    MyHashSet() {
    
    
        int max=1e6,num=max/base;
        if(num*base==max)
            ++num;
        hashTable.resize(num);
    }
    
    void add(int key) {
    
    
        hashTable[key/base]|=1ull<<(key%base);
    }
    
    void remove(int key) {
    
    
        hashTable[key/base]&=~(1ull<<(key%base));
    }
    
    /** Returns true if this set contains the specified element */
    bool contains(int key) {
    
    
        return hashTable[key/base]&(1ull<<(key%base));
    }
};

/**
 * Your MyHashSet object will be instantiated and called as such:
 * MyHashSet* obj = new MyHashSet();
 * obj->add(key);
 * obj->remove(key);
 * bool param_3 = obj->contains(key);
 */