## Tuesday, August 9, 2016

### LeetCode 381 - Insert Delete GetRandom O(1) - Duplicates allowed

https://www.hrwhisper.me/leetcode-insert-delete-getrandom-o1-duplicates-allowed/
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();
```

private:
unordered_map<int, vector<int>> index;
vector<int> output;
public:
/** Initialize your data structure here. */
RandomizedCollection() {}

/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
bool insert(int val) {
bool return_val = index.find(val) == index.end();
index[val].push_back(output.size());
output.push_back(val);
return return_val;
}

/** Removes a value from the collection. Returns true if the collection contained the specified element. */
bool remove(int val) {
if (index.find(val) == index.end()) return false;
int _id = index[val].back(); index[val].pop_back();
int last = output.back(); output.pop_back();
if (last != val) {
index[last].back() = _id;
output[_id] = last;
}
if (index[val].empty())
index.erase(val);
return true;
}

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