Question Definition
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();
### Java Solution
```java
class RandomizedSet {
Map<Integer, Integer> map;
List<Integer> list;
Random random;
/** Initialize your data structure here. */
public RandomizedSet() {
map = new HashMap<>();
list = new ArrayList<>();
random = new Random();
}
/** 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)) return false;
map.put(val, list.size());
list.add(val);
return true;
}
/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
if(!map.containsKey(val)) return false;
list.set(map.get(val), list.get(list.size() - 1));
map.put(list.get(list.size() - 1), map.get(val));
map.remove(val);
list.remove(list.size() - 1);
return true;
}
/** Get a random element from the set. */
public int getRandom() {
return list.get(random.nextInt(list.size()));
}
}
Comments