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.

insert(val): Inserts an item val to the collection.
remove(val): Removes an item val from the collection if present.
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：

 1 public class RandomizedCollection {
 2     HashMap<Integer,Set<Integer>> map;
 3     List<Integer> nums;
 4     java.util.Random rand = new java.util.Random();
 5     /** Initialize your data structure here. */
 6     public RandomizedCollection() {
 7         map = new HashMap<>();
 8         nums = new ArrayList<Integer>();
 9     }
10     
11     /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
12     public boolean insert(int val) {
13         boolean contain = map.containsKey(val);
14         if(!contain){
15             Set<Integer> set = new LinkedHashSet<Integer>();
16             map.put(val,set);
17         }
18         map.get(val).add(nums.size());
19         nums.add(val);
20         return !contain;
21     }
22     
23     /** Removes a value from the collection. Returns true if the collection contained the specified element. */
24     public boolean remove(int val) {
25         boolean contain = map.containsKey(val);
26         if(!contain) return false;
27         int loc = map.get(val).iterator().next();
28         map.get(val).remove(loc);
29         if(loc<nums.size()-1){
30             int lastone = nums.get(nums.size()-1);
31             nums.set(loc,lastone);
32             map.get(lastone).remove(nums.size()-1);
33             map.get(lastone).add(loc);
34         }
35         nums.remove(nums.size()-1);
36         if(map.get(val).isEmpty()) map.remove(val);
37         return true;
38     }
39     
40     /** Get a random element from the collection. */
41     public int getRandom() {
42         return nums.get(rand.nextInt(nums.size()));
43     }
44 }
45 
46 /**
47  * Your RandomizedCollection object will be instantiated and called as such:
48  * RandomizedCollection obj = new RandomizedCollection();
49  * boolean param_1 = obj.insert(val);
50  * boolean param_2 = obj.remove(val);
51  * int param_3 = obj.getRandom();
52  */