[LeetCode] 380. Insert Delete GetRandom O(1)

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);
    }
}

Allow Duplicates

LC381
https://segmentfault.com/a/11...