LeetCode 381. Insert Delete GetRandom O - Duplicates allowed (插入删除和获得随机数 常数时间 允许重复项)

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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();

 

 


题目标签:Array, Hash Table, Design
  这道题目基于#380 的情况下,可以允许有重复项。回顾一下380, 因为要达到 insert, remove 和 getRandom 都是O(1) 的时间,我们需要ArrayList 来保存所有的数字,利用map 来保存 数字 和 index 之间的映射。
  这道题目允许了重复项,那么在map 里 key 是数字, value 是index, 这里的index 就会不止一个了。我们要把所有重复的数字的 index 也保存进来, 所以把 map 里value 改成 HashSet 来保存所有的index。
 
  insert val:如果map里没有val,需要新建一个HashSet,并且加入nums (ArrayList);
         如果有val,直接加入新的index 进HashSet,并且加入nums (ArrayList);  
        
  remove val:如果val 在nums 里是最后一个的话,只需要在map 里删除val 的index, 并且在nums 里删除最后一个数字。
        如果val 在nums 里不是最后一个的话,需要额外的把 nums 里最后一个数字的值 复制到 val, 删除val 在map里的 index,还要把最后一个数字的index 在map 里更新,并且在nums 里删除最后一个数字。
 
  其他基本都和#380 差不多,具体看code。
 

Java Solution:

Runtime beats 88.45% 

完成日期:09/18/2017

关键词:Array, Hash Table, Design

关键点:利用array 保存数值;利用map<Integer, HashSet<>>保存 - 数值 当作key,数值在array里的所有index 保存在HashSet,当作value。

 1 class RandomizedCollection 
 2 {
 3     private HashMap<Integer, HashSet<Integer>> map; // key is value, value is index HashSet
 4     private ArrayList<Integer> nums; // store all vals
 5     private java.util.Random rand = new java.util.Random();
 6     
 7     /** Initialize your data structure here. */
 8     public RandomizedCollection() 
 9     {
10         map = new HashMap<>();
11         nums = new ArrayList<>();
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     {
17         boolean contain = map.containsKey(val);
18         
19         // if map doesn\'t have val, meaning map doesn\'t have HashSet
20         if(!contain) 
21             map.put(val, new HashSet<Integer>()); // create HashSet
22         
23         // add index into HashSet
24         map.get(val).add(nums.size());
25         nums.add(val);
26         
27         return !contain; // if collection has val, return false; else return true
28     }
29     
30     /** Removes a value from the collection. Returns true if the collection contained the specified element. */
31     public boolean remove(int val) 
32     {
33         boolean contain = map.containsKey(val);
34         if(!contain) 
35             return false;
36         // get an index from HashSet of Map
37         int valIndex = map.get(val).iterator().next();
38         map.get(val).remove(valIndex); // remove this index from val\'s set
39         
40         if(valIndex != nums.size() - 1) // if this val is not the last one in nums
41         {
42             // copy the last one value into this val\'s position
43             int lastNum = nums.get(nums.size() - 1);
44             nums.set(valIndex, lastNum);
45             // update the lastNum index in HashSet
46             map.get(lastNum).remove(nums.size() - 1); // remove the last number\'s index from set
47             map.get(lastNum).add(valIndex); // add new index into set
48         }
49         
50         if(map.get(val).isEmpty()) // if val\'s set is empty
51             map.remove(val); // remove val from map
52         
53         nums.remove(nums.size() - 1); // only remove last one O(1)
54           
55         return true;
56     }
57     
58     /** Get a random element from the collection. */
59     public int getRandom() 
60     {
61         return nums.get(rand.nextInt(nums.size()));
62     }
63 }
64 
65 /**
66  * Your RandomizedCollection object will be instantiated and called as such:
67  * RandomizedCollection obj = new RandomizedCollection();
68  * boolean param_1 = obj.insert(val);
69  * boolean param_2 = obj.remove(val);
70  * int param_3 = obj.getRandom();
71  */

参考资料:

https://discuss.leetcode.com/topic/53216/java-solution-using-a-hashmap-and-an-arraylist-along-with-a-follow-up-131-ms/5

 

LeetCode 题目列表 - LeetCode Questions List
 

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