381. 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();

本题其实不是很难，只是有一个数据结构需要注意一下，就是linkedhashset，它是链表和hashset的结合，本身带有顺序性，代码如下：

public class RandomizedCollection {
    HashMap<Integer,Set<Integer>> map;
    List<Integer> nums;
    java.util.Random rand = new java.util.Random();
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        map = new HashMap<>();
        nums = new ArrayList<Integer>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean contain = map.containsKey(val);
        if(!contain){
            Set<Integer> set = new LinkedHashSet<Integer>();
            map.put(val,set);
        }
        map.get(val).add(nums.size());
        nums.add(val);
        return !contain;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        boolean contain = map.containsKey(val);
        if(!contain) return false;
        int loc = map.get(val).iterator().next();
        map.get(val).remove(loc);
        if(loc<nums.size()-1){
            int lastone = nums.get(nums.size()-1);
            nums.set(loc,lastone);
            map.get(lastone).remove(nums.size()-1);
            map.get(lastone).add(loc);
        }
        nums.remove(nums.size()-1);
        if(map.get(val).isEmpty()) map.remove(val);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.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();
 */