[leetcode] 380. Insert Delete GetRandom O

Posted 2020-08-13

Design a data structure that supports all following operations in average O(1) time.

 

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

 

Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

 

 

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Solution:

 1 class RandomizedSet {
 2 public:
 3     /** Initialize your data structure here. */
 4     RandomizedSet() 
 5     {
 6     }
 7     
 8     /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
 9     bool insert(int val) 
10     {
11         if (hset.find(val) != hset.end()) // exist
12             return false;
13         hset.insert(val);
14         array.push_back(val);
15         return true;
16     }
17     
18     /** Removes a value from the set. Returns true if the set contained the specified element. */
19     bool remove(int val) 
20     {
21         if (hset.find(val) == hset.end()) // not exist
22             return false;
23         unordered_set<int>::iterator sit = hset.find(val);
24         hset.erase(sit);
25         
26         vector<int>::iterator vit = array.begin();
27         while (vit != array.end())
28         {
29             if (*vit == val) // erase it
30             {
31                 vit = array.erase(vit);
32             }
33             else
34             {
35                 vit++;
36             }
37         }
38         
39         return true;
40     }
41     
42     /** Get a random element from the set. */
43     int getRandom() 
44     {
45         return array[rand() % array.size()];
46     }
47     
48 private:
49     vector<int> array;
50     unordered_set<int> hset;
51 };