Design HashMap

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.

Analysis

注意Hash Function和Collision Resolution

Solution

import javafx.util.Pair;

class MyHashMap {
    private final int MAX_LEN = 100000;             // the amount of buckets
    private List<Pair<Integer, Integer>>[] map;     // hash map implemented by array

    /** Returns the corresponding bucket index. */
    private int getIndex(int key) {
        return key % MAX_LEN;
    }

    /** Search the key in a specific bucket. Returns -1 if the key does not existed. */
    private int getPos(int key, int index) {
        // Each bucket contains a list.
        List<Pair<Integer, Integer>> temp = map[index];
        if (temp == null) {
            return -1;
        }
        // Iterate all the elements in the bucket to find the target key.
        for (int i = 0; i < temp.size(); ++i) {
            if (temp.get(i).getKey() == key) {
                return i;
            }
        }
        return -1;
    }

    /** Initialize your data structure here. */
    public MyHashMap() {
        map = (List<Pair<Integer, Integer>>[])new ArrayList[MAX_LEN];
    }

    /** value will always be positive. */
    public void put(int key, int value) {
        int index = getIndex(key);
        int pos = getPos(key, index);
        if (pos < 0) {
            // Add new (key, value) pair if key is not existed.
            if (map[index] == null) {
                map[index] = new ArrayList<Pair<Integer, Integer>>();
            }
            map[index].add(new Pair(key, value));
        } else {
            // Update the value if key is existed.
            map[index].set(pos, new Pair(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) {
        int index = getIndex(key);
        int pos = getPos(key, index);
        if (pos < 0) {
            return -1;
        } else {
            return map[index].get(pos).getValue();
        }
    }

    /** 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);
        int pos = getPos(key, index);
        if (pos >= 0) {
            map[index].remove(pos);
        }
    }
}

/**
 * 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);
 */