380. Insert Delete GetRandom O(1)
Design a data structure that supports all following operations in average O(1) time.
Note: Duplicate elements are allowed.
insert(val): Inserts an item val to the collection. remove(val): Removes an item val from the collection if present. 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();
1. Analyse
Same principle as 380. Insert Delete GetRandom O(1). But this time we store a struct into unordered_map.
AC Code
class RandomizedCollection {
public:
vector<int> position;
unordered_map<int, unordered_set<int> > mp;
/** Initialize your data structure here. */
RandomizedCollection() {
srand(time(NULL));
}
/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
bool insert(int val) {
mp[val].insert( position.size() );
position.push_back(val);
return 1 == mp[val].size();
}
/** Removes a value from the collection. Returns true if the collection contained the specified element. */
bool remove(int val) {
if( 0 == mp.count(val) ) return false;
int index = *mp[val].begin();
int last_item = position.back();
position[ index ] = last_item; //move the last item
mp[val].erase( index ); //remove val index
mp[last_item].insert( index ); //add last_item index
mp[last_item].erase( position.size()-1 ); //remove last_item index
position.pop_back(); //remove the last item
if( mp[val].empty() )
mp.erase( val ); //remove val index
return true;
}
/** Get a random element from the collection. */
int getRandom() {
return position[ rand() % position.size() ];
}
};
