[leetcode]381. Insert Delete GetRandom O(1) - Duplicates allowed常数时间插入删除取随机值

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;
     }
 }