Design a data structure that supports all following operations in averageO(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 { private Map<Integer, Integer> map; private List<Integer> list; /** Initialize your data structure here. */ public RandomizedSet() { map = new HashMap<>(); list = new ArrayList<>(); }
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ public boolean insert(int val) { if (!map.containsKey(val)) { int index = list.size(); map.put(val, index); list.add(val); return true; } return false; }
/** Removes a value from the set. Returns true if the set contained the specified element. */ public boolean remove(int val) { if (map.containsKey(val)) { int index = map.get(val); int lastIndex = list.size() - 1; int valAtlastIndex = list.get(lastIndex); Collections.swap(list, index, lastIndex); map.put(valAtlastIndex, index); list.remove(lastIndex); map.remove(val); return true; } return false; }
/** Get a random element from the set. */ public int getRandom() { int rand = new Random().nextInt(list.size()); return list.get(rand); } }
/** * Your RandomizedSet object will be instantiated and called as such: * RandomizedSet obj = new RandomizedSet(); * boolean param_1 = obj.insert(val); * boolean param_2 = obj.remove(val); * int param_3 = obj.getRandom(); */