380. Insert Delete GetRandom O(1) (Medium)
https://leetcode.com/problems/insert-delete-getrandom-o1/
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();
Solutions
class RandomizedSet {
List<Integer> values = new ArrayList<>();
Set<Integer> presentValues = new HashSet<>();
/** Initialize your data structure here. */
public RandomizedSet() {
}
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
if (!presentValues.add(val)) {
return false;
}
values.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 (!presentValues.remove(val)) {
return false;
}
// it must be converted to object type. primitive int represents the index, not the element.
values.remove(Integer.valueOf(val));
return true;
}
/** Get a random element from the set. */
public int getRandom() {
int len = presentValues.size();
// index should not be len + 1 because it already overstep the boundary of the list.
int ridx = new Random().nextInt(len);
return values.get(ridx);
}
}