380.insert-delete-get-random-o-1.java

/*
 * @lc app=leetcode id=380 lang=java
 *
 * [380] Insert Delete GetRandom O(1)
 */

// @lc code=start
class RandomizedSet {
    Map<Integer, Integer> map;
    List<Integer> list;
    Random rand = new Random();

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
    }
    
    /** 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;
        }
        map.put(val, list.size());
        list.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 (!map.containsKey(val)) {
            return false;
        }
        int lastElement = list.get(list.size() - 1);
        int index = map.get(val);
        //replace the deleting element with last element
        list.set(index, lastElement);
        map.put(lastElement, index);
        //delete the element
        list.remove(list.size() - 1);
        map.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rand.nextInt(list.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();
 */
// @lc code=end