706. Design HashMap

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.

M1: 直接用数组，大小为1000000

class MyHashMap {
    int[] map;

    /** Initialize your data structure here. */
    public MyHashMap() {
        map = new int[1000001];
        Arrays.fill(map, -1);
    }
    
    /** value will always be non-negative. */
    public void put(int key, int value) {
        map[key] = value;
    }
    
    /** Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key */
    public int get(int key) {
        return map[key];
    }
    
    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    public void remove(int key) {
        map[key] = -1;
    }
}

/**
 * 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);
 */

M2: 用较小的数组，linked list解决hash collision

ref: https://leetcode.com/problems/design-hashmap/discuss/152746/Java-Solution

class MyHashMap {
    class ListNode {
        int val;
        int key;
        ListNode next;
        public ListNode(int k, int v) {
            this.key = k;
            this.val = v;
        }
    }
    ListNode[] nodes;

    /** Initialize your data structure here. */
    public MyHashMap() {
        nodes = new ListNode[10000];
    }
    
    /** value will always be non-negative. */
    public void put(int key, int value) {
        int i = Integer.hashCode(key) % nodes.length;
        if(nodes[i] == null) {
            nodes[i] = new ListNode(-1, -1);
        }
        ListNode prev = find(nodes[i], key);
        if(prev.next == null) {
            prev.next = new ListNode(key, value);
        } else {
            prev.next.val = value;
        }
    }
    
    private ListNode find(ListNode node, int key) {
        ListNode n = node, prev = null;
        while(n != null && n.key != key) {
            prev = n;
            n = n.next;
        }
        return prev;
    }
    
    /** Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key */
    public int get(int key) {
        int idx = Integer.hashCode(key) % nodes.length;
        if(nodes[idx] == null) {
            return -1;
        }
        ListNode node = find(nodes[idx], key);
        return node.next == null ? -1 : node.next.val;
    }
    
    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    public void remove(int key) {
        int idx = Integer.hashCode(key) % nodes.length;
        if(nodes[idx] == null) {
            return;
        }
        ListNode prev = find(nodes[idx], key);
        if (prev.next == null) {
            return;
        }
        prev.next = prev.next.next;
    }
}

/**
 * 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);
 */