RandomizedSet.java

public class RandomizedSet {
    /** Initialize your data structure here. */
    ArrayList<Integer> nums;
    HashMap<Integer, Integer> locs;
    java.util.Random rand = new java.util.Random();
    public RandomizedSet() {
        nums = new ArrayList<Integer>();
        locs = new HashMap<Integer, Integer>();
    }
    
    public boolean insert(int val) {
        if (locs.containsKey(val)) 
            return false;
        locs.put( val, nums.size());
        nums.add(val);
        return true;
    }
    
    public boolean remove(int val) {
        if(!locs.containsKey(val)) 
            return false;
        int loc = locs.get(val);
        if (loc < nums.size() - 1 ) {
            int lastone = nums.get(nums.size() - 1 );
            nums.set( loc , lastone );
            locs.put(lastone, loc);
        }
        locs.remove(val);
        nums.remove(nums.size() - 1);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return nums.get( rand.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();
 */