leetcode381- Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

算法：试了一下用双Map无法实现频率正比概率random。当然如果random的概率不和频率成正比而是等概率的话反而还真的必须得用双map。

最后使用Map + List。和前面的区别在于Map<Integer, Set<Integer>>，维护的映射从存储单个int下标信息变成了用一个Set存储下标信息。 删的时候比较麻烦。细节不写好容易报错。

1.删待删数对应的一个下标（如果set空了记得清空整个key）。

2.如果之前删的下标不是最后一个，那你还要把最后一个数提到下标那里（更新该val的下标信息要一加一删，切记），然后在list里也更新一下。

3.在List里把最后一个数删掉，size更新。返回

实现：

class RandomizedCollection {
    
    private List<Integer> list;
    private Map<Integer, Set<Integer>> vi;
    private int size;
    
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        list = new ArrayList<Integer>();
        vi = new HashMap<Integer, Set<Integer>>();
        size = 0;
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean contains = vi.containsKey(val);
        if (!contains) {
            vi.put(val, new LinkedHashSet<>());
        }
        vi.get(val).add(size);
        list.add(val);
        size++;
        // 注意是返回!contains!，相反的
        return !contains;
    }
    
     
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if (!vi.containsKey(val)) {
            return false;
        }
        
        // 删原来那个的下标
        int index = vi.get(val).iterator().next();
        vi.get(val).remove(index);
        if (vi.get(val).size() == 0) {
            vi.remove(val);
        }    
        
        // 如果需要换的话把换到前面后的下标信息更新
        if (index < size - 1) {
            int lastVal = list.get(size - 1);
            //这句不要漏了啊啊啊！更新最后一个的坐标要一加一删
            vi.get(lastVal).remove(size - 1);
            vi.get(lastVal).add(index);
            list.set(index, lastVal);    
        }
        
        list.remove(size - 1);
        
        size--;
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(new Random().nextInt(size));
    }
}

/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * RandomizedCollection obj = new RandomizedCollection();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */