[leetcode]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();

Solution1： HashMap + ArrayList

  

code

public class RandomizedCollection {
    class Node {
        public int value;
        public int index;
        public Node(int val, int idx) {
            value = val;
            index = idx;
        }
    }

    private Map<Integer, List<Integer>> map;
    private List<Node> list;
    private Random r;

    /** Initialize your data structure here. */
    public RandomizedCollection() {
        map = new HashMap<>();
        list = new ArrayList<>();
        r = new Random();
    }

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        List<Integer> l = map.getOrDefault(val, new ArrayList<>());
        l.add(list.size());
        map.put(val, l);
        list.add(new Node(val, l.size() - 1));
        return l.size() == 1;
    }

    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) return false;
        List<Integer> l = map.get(val);
        int removeIdx = l.get(l.size() - 1);
        Node replaceNode = list.get(list.size() - 1);

        // deal with HashMap
         map.get(replaceNode.value).set(replaceNode.index, removeIdx);
         l.remove(l.size() - 1);
         if (l.size() == 0) map.remove(val);

        // deal with List
        list.set(removeIdx, replaceNode);
        list.remove(list.size() - 1);
        
        return true;
    }

    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(r.nextInt(list.size())).value;
    }
}