705. Design HashSet

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

To be specific, your design should include these functions:

• add(value): Insert a value into the HashSet. 
• contains(value) : Return whether the value exists in the HashSet or not.
• remove(value): Remove a value in the HashSet. If the value does not exist in the HashSet, do nothing.

Example:

MyHashSet hashSet = new MyHashSet();
hashSet.add(1);         
hashSet.add(2);         
hashSet.contains(1);    // returns true
hashSet.contains(3);    // returns false (not found)
hashSet.add(2);          
hashSet.contains(2);    // returns true
hashSet.remove(2);          
hashSet.contains(2);    // returns false (already removed)

Note:

• All 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 HashSet library.

class MyHashSet {
    
    List<Integer> list;
    /** Initialize your data structure here. */
    public MyHashSet() {
        list = new ArrayList();
    }
    
    public void add(int key) {
        if(list.indexOf(key) < 0) list.add((Integer) key);
    }
    
    public void remove(int key) {
        if(list.indexOf(key) >= 0) list.remove((Integer) key);
    }
    
    /** Returns true if this set contains the specified element */
    public boolean contains(int key) {
        return list.indexOf((Integer) key) >= 0;
    }
}

1. 相当于bruteforce了

class MyHashSet {
    boolean[] arr = new boolean[100];// start with 100 elements for fast initialization
    /** Initialize your data structure here. */
    public MyHashSet() {
        
    }
    
    public void add(int key) {
        if(key>=arr.length) // if array is too small to accomodate key, extend it.
            extend(key);
        arr[key]=true;
    }
    
    public void remove(int key) {
        if(key>=arr.length) // if array is too small to accomodate key, extend it.
            return;
        arr[key]=false;
    }
    
    /** Returns true if this set contains the specified element */
    public boolean contains(int key) {
        if(key>=arr.length) // key cannot be in array if array's length < key
            return false;
        return arr[key]==true;
    }
    
    public void extend(int key){
        arr= Arrays.copyOf(arr, key+1);  // extend array to one more item than necessary, we need "key" items. 
                                         // we give "key+1" items to reduce collisions.
    }
}

2.相当于里面有了检查的阈值