1. Runtime Distribution

2. Submission Details

3. Description

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.

What is the maximum number of envelopes can you Russian doll? (put one inside other)

4. 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();

5. Code

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private Map map;
private List list;

public RandomizedSet() {
	map = new HashMap();
	list = new ArrayList();

}

public boolean insert(int val) {
	if (map.containsKey(val)) {
		return false;
	}
	map.put(val, list.size());
	list.add(val);
	return true;
}

public boolean remove(int val) {
	if (!map.containsKey(val)) {
		return false;
	}
	int index = map.get(val);
	int lastIndex = list.size() - 1;
	list.set(index, list.get(lastIndex));
	map.put(list.get(lastIndex), index);
	list.remove(lastIndex);
	map.remove(val);
	return true;
}

public int getRandom() {
	return list.get((int) (Math.random() * list.size()));
}

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6.Test

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import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class LeetCode0380 {

	private Map map;
	private List list;

	public LeetCode0380() {
		map = new HashMap();
		list = new ArrayList();

	}

	public boolean insert(int val) {
		if (map.containsKey(val)) {
			return false;
		}
		map.put(val, list.size());
		list.add(val);
		return true;
	}

	public boolean remove(int val) {
		if (!map.containsKey(val)) {
			return false;
		}
		int index = map.get(val);
		int lastIndex = list.size() - 1;
		list.set(index, list.get(lastIndex));
		map.put(list.get(lastIndex), index);
		list.remove(lastIndex);
		map.remove(val);
		return true;
	}

	public int getRandom() {
		return list.get((int) (Math.random() * list.size()));
	}
}

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