Problem
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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();
Solution
class RandomizedSet {
Map<Integer, Integer> map;
List<Integer> list;
public RandomizedSet() {
map = new HashMap<>();
list = new ArrayList<>();
}
public boolean insert(int val) {
if (map.containsKey(val)) return false;
list.add(val);
map.put(val, list.size()-1);
return true;
}
public boolean remove(int val) {
int index = map.getOrDefault(val, -1);
if (index == -1) return false;
//swap val with the tail element in list
Collections.swap(list, index, list.size()-1);
//update the swapped tail element in map
map.put(list.get(index), index);
//remove val from the tail of list, remove val from map
list.remove(list.size()-1);
map.remove(val);
return true;
}
public int getRandom() {
int max = list.size();
int min = 0;
int index = (int)(Math.random() * (max-min) + min);
return list.get(index);
}
}