Leetcode 380 - insert, delete, getRandom() O(1) - study code - from blog: http://www.guoting.org/leetcode/leetcode-380-insert-delete-getrandom-o1/

Leetcode380_studyCode1.java

public class RandomizedSet {

    private List<Integer> list;
    private Map<Integer,Integer> map;
    java.util.Random rand = new java.util.Random();
    /** 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;
        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 index=map.get(val);
        if(index!=list.size()-1){
            int last=list.get(list.size()-1);
            list.set(index,last);
            map.put(last,index);
        }
        map.remove(val);
        list.remove(list.size()-1);
        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();
 */