Insert Delete GetRandom O(1) - Duplicates allowed

Desicription

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:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
// 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();

Solution

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
class RandomizedCollection {
private:
std::unordered_map<int, std::vector<int>> map = std::unordered_map<int, std::vector<int>>{};
std::vector<std::pair<int, int>> vector = std::vector<std::pair<int, int>>{};
public:
/** Initialize your data structure here. */
RandomizedCollection() = default;

/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
bool insert(int val) {
auto result = map.find(val) == map.end();
map[val].push_back(vector.size());
vector.push_back({val, map[val].size() - 1});

return result;
}

/** Removes a value from the collection. Returns true if the collection contained the specified element. */
bool remove(int val) {
auto result = map.find(val) != map.end();

if(result) {
auto last = vector.back();
map[last.first][last.second] = map[val].back();
vector[map[val].back()] = last;
map[val].pop_back();
if(map[val].empty()) {
map.erase(val);
}
vector.pop_back();
}

return result;
}

/** Get a random element from the collection. */
int getRandom() {
return vector[rand() % vector.size()].first;
}
};

/**
* Your RandomizedCollection object will be instantiated and called as such:
* RandomizedCollection* obj = new RandomizedCollection();
* bool param_1 = obj->insert(val);
* bool param_2 = obj->remove(val);
* int param_3 = obj->getRandom();
*/