380. Insert Delete GetRandom O(1)
Design a data structure that supports all following operations in average O(1) time.
insert(val): Inserts an item val to the set if not already present. remove(val): Removes an item val from the set if present. 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();
1. Analyse
O(1) insert and remove is easy, we can use a set. But getRandom is not O(1) in set. getRandom is O(1) in vector, but remove is not O(1) in vector.
If we use vector as the data structure, getRandom in O(1) is easy. Insert in O(1) is easy. How can we do remove in O(1)?
The reason why remove in O(1) is hard is that we don’t konw the target position. If we store the position information, we know the target position. But remove operation in vecotor will move all the elements after the target. But why don’t we swap the element to the end of the vector? :)
2. AC Code
(32 Lines && 12 Comments)
class RandomizedSet {
public:
vector<int> vec;
unordered_map<int, int> mp;
/** Initialize your data structure here. */
RandomizedSet() {
srand(time(NULL));
}
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
if( mp.find(val) == mp.end() ){
mp[val] = vec.size();
vec.push_back( val );
return true;
}
return false;
}
/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
if( vec.size() > 0 && mp.find(val) != mp.end() ){
vec[mp[val]] = vec.back();
mp[vec.back()] = mp[val];
vec.pop_back();
mp.erase(val);
return true;
}
return false;
}
/** Get a random element from the set. */
int getRandom() {
return vec[rand() % vec.size()];
}
};
/**
* Your RandomizedSet object will be instantiated and called as such:
* RandomizedSet obj = new RandomizedSet();
* bool param_1 = obj.insert(val);
* bool param_2 = obj.remove(val);
* int param_3 = obj.getRandom();
*/
