Leetcode 380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time.

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. 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();

Analysis:

We can use a hashMap + array here. 

Insert - insert the val to the tail of the array, and update the hash map.

Remove - swap the val to be deleted with the tail of the list node. Update the index in the hash map, and remove tail of the array

GetRandom - Get a random index in the array and return its value.

Code (Java):

class RandomizedSet {
    private List<Integer> list;
    private Map<Integer, Integer> map;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        map = new HashMap<>();
    }
    
    /** 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;
        }
        
        int index = list.size();
        list.add(val);
        map.put(val, index);
        
        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;
        }
        
        int indexRemove = map.get(val);
        int tail = list.get(list.size() - 1);
        
        swap(indexRemove, list.size() - 1);
        map.put(tail, indexRemove);
        list.remove(list.size() - 1);
        map.remove(val);
        
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        
        if (list.isEmpty()) {
            return 0;
        }
        
        Random rand = new Random();
        int index = rand.nextInt(list.size());
        
        return list.get(index);
    }
    
    private void swap(int i, int j) {
        int temp = list.get(i);
        list.set(i, list.get(j));
        list.set(j, temp);
    }
}

/**
 * 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();
 */