Leetcode380-insertDeleteGetRandomO1

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.

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 {

    HashMap<Integer, Integer> map;
    List<Integer> nums;
    Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        nums = 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)){
            nums.add(val);
            map.put(val,nums.size()-1);
            return true;
        }
        else {
            return false;
        }
    }
    
    /** 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;
        else {
            int index = map.get(val);
            
            if (index < nums.size()-1){
                nums.set(index, nums.get(nums.size()-1));
                map.put(nums.get(nums.size()-1), index);
            }
            nums.remove(nums.size()-1);
            map.remove(val);
            return true;
        }
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return nums.get(random.nextInt(nums.size()));
    }
}

/**
 * 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();
 */

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