[Leetcode] 设计哈希映射

不使用任何内建的哈希表库设计一个哈希映射
具体地说，你的设计应该包含以下的功能

• put(key, value)：向哈希映射中插入(键,值)的数值对。如果键对应的值已经存在，更新这个值。
• get(key)：返回给定的键所对应的值，如果映射中不包含这个键，返回-1。
• remove(key)：如果映射中存在这个键，删除这个数值对。

public class MyHashMap {
    static final int DEFAULT_INITIAL_CAPACITY = 16;
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    static final int MAXIMUM_CAPACITY = 1 << 30;

    Entry[] table;
    int size;    
    int threshold;
    float loadFactor;
    
    /** Initialize your data structure here. */
    public MyHashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        this.threshold = (int) (DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
        this.table = new Entry[DEFAULT_INITIAL_CAPACITY];
    }
    
    /** value will always be non-negative. */
    public void put(int key, int value) {
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry e = table[i]; e != null; e = e.next) {
            if (e.hash == hash) {
                e.value = value;
                return;
            }
        }
        Entry e = table[i];
        table[i] = new Entry(hash, key, value, e);
        if (size >= threshold)
            resize(table.length << 1);
    }
    
    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }
        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int) (newCapacity * loadFactor);
    }
    
    void transfer(Entry[] newTable) {
        Entry[] src = table;
        int newCapacity = newTable.length;
        for (int j = 0; j < src.length; j++) {
            Entry e = src[j];
            if (e != null)
                src[j] = null;
            do {
                Entry next = e.next;
                int i = indexFor(e.hash, newCapacity);
                e.next = newTable[i];
                newTable[i] = e;
                e = e.next;
            } while (e != null);
        }
    }
    
    /** 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 hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry e = table[i]; e != null; e = e.next) {
            if (e.hash == hash)
                return e.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 hash = hash(key);
        int i = indexFor(hash, table.length);
        Entry prev = table[i];
        Entry e = prev;
        while (e != null) {
            Entry next = e.next;
            if (e.hash == hash) {
                size--;
                if (prev == e) 
                    table[i] = next;
                else
                    prev.next = next;
                return;
            }
            prev = e;
            e = next;
        }
    }
    
    static int hash(int h) {
        return h ^ (h >>> 16);
    }
    
    static int indexFor(int h, int length) {
        return h & (length - 1);
    }
    
    static class Entry {
        int key;
        int value;
        Entry next;
        int hash;
        
        Entry(int hash, int key, int value, Entry next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
    }
}