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();

O(1) 时间插入、删除和获取随机元素 - 允许重复。题意跟380题很接近，唯一不同的条件是这个题允许加入相同元素。

思路还是用hashmap + arrayList，hashmap的key是每个不同的数字，value是一个hashset，存的是每个数字在list里的index。

insert() - 遇到相同的数字的时候，每次记得先在hashmap的key中加入list当前的size再处理list的部分。arrayList还是保存每一个被加入的num。

remove() - 从hashmap里删除的时候，如果某个元素出现过多次，则删除一次就好，此时需要对hashset做iterate；如果这个元素只出现了一次，记得要将hashmap中的key也一并删除。对list做删除操作的时候，跟380题一样，也是需要把list的最后一个元素移动到list里被删除的元素的位置上。

时间O(1) - required

空间O(n)

Java实现

 1 class RandomizedCollection {
 2 
 3     HashMap<Integer, HashSet<Integer>> map;
 4     List<Integer> list;
 5     Random rmd;
 6 
 7     /** Initialize your data structure here. */
 8     public RandomizedCollection() {
 9         map = new HashMap<>();
10         list = new ArrayList<>();
11         rmd = new Random();
12     }
13 
14     /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
15     public boolean insert(int val) {
16         boolean contain = map.containsKey(val);
17         if (!contain) {
18             map.put(val, new HashSet<>());
19         }
20         map.get(val).add(list.size());
21         list.add(val);
22         return !contain;
23     }
24 
25     /** Removes a value from the collection. Returns true if the collection contained the specified element. */
26     public boolean remove(int val) {
27         if (!map.containsKey(val)) {
28             return false;
29         }
30         int index = map.get(val).iterator().next();
31         map.get(val).remove(index);
32         if (map.get(val).size() == 0) {
33             map.remove(val);
34         }
35         int lastVal = list.remove(list.size() - 1);
36         if (index != list.size()) {
37             list.set(index, lastVal);
38             map.get(lastVal).remove(list.size());
39             map.get(lastVal).add(index);
40         }
41         return true;
42     }
43 
44     /** Get a random element from the collection. */
45     public int getRandom() {
46         return list.get(rmd.nextInt(list.size()));
47     }
48 }
49 
50 /**
51  * Your RandomizedCollection object will be instantiated and called as such:
52  * RandomizedCollection obj = new RandomizedCollection();
53  * boolean param_1 = obj.insert(val);
54  * boolean param_2 = obj.remove(val);
55  * int param_3 = obj.getRandom();
56  */

 

相关题目

380. Insert Delete GetRandom O(1)

381. Insert Delete GetRandom O(1) - Duplicates allowed

LeetCode 题目总结