Friday, January 29, 2021

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.

My answer:

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class MyHashMap {

    // array + chaining, I missed case: existing key is updated
    // use array, and each element is a list, containing nodes, each of node has key and value associated
    
    class Node {
        public int key;
        public int value;
        Node (int key, int value) {
            this.key = key;
            this.value = value;
        }
    }
    
    class ListNode {
        public List<Node> nodes;
        public ListNode () {
            nodes = new ArrayList<Node>();
        }
    }
    
    private ListNode[] elements;
    /** Initialize your data structure here. */
    public MyHashMap() {
         elements = new ListNode[10000];
    }
    
    private int getIndex(int key) {
        return key % elements.length;
    }
    
    /** value will always be non-negative. */
    public void put(int key, int value) {
        int index = getIndex(key);
        ListNode nodeList = elements[index]; 
        if (nodeList == null) {
            nodeList = new ListNode();
        }
        boolean existed = false;
        for (Node node : nodeList.nodes) {
            if (node.key == key) {
                node.value = value;
                existed = true;
            }
        }
        if (!existed) {
            nodeList.nodes.add(new Node(key, value));    
        }

        elements[index] = nodeList;
    }
    
    /** 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 index = getIndex(key);
        ListNode nodeList = elements[index];
        if (nodeList == null) {
            return -1;
        }
        for (Node node : nodeList.nodes) {
            if (node.key == key) {
                return node.value;
            }
        }
        return -1;
    }
    
    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    public void remove(int key) {
        int index = getIndex(key);
        ListNode nodeList = elements[index];
        if (nodeList == null || nodeList.nodes.isEmpty()) {
            return;
        }
        int nodeIndex = -1;
        for (int i = 0; i < nodeList.nodes.size(); i ++) {
            if (nodeList.nodes.get(i).key == key) {
                nodeIndex = i;
                break;
            }
        }
        if (nodeIndex != -1) {
            nodeList.nodes.remove(nodeIndex);
        }
    }
}

/**
 * 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);
 */


 

at

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