insert-delete-getrandom-o1-duplicates-allowed

https://leetcode.com/problems/insert-delete-getrandom-o1-duplicates-allowed/

public class RandomizedCollection {
    ArrayList<Integer> nums;
    HashMap<Integer, Set<Integer>> locs;
    java.util.Random rand;

    /** Initialize your data structure here. */
    public RandomizedCollection() {
        rand = new java.util.Random();
        nums = new ArrayList<Integer>();
        locs = new HashMap<Integer, Set<Integer>>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean contained = locs.containsKey(val);
        if (!contained) {
            locs.put(val, new LinkedHashSet<Integer>());
        }
        locs.get(val).add(nums.size());
        nums.add(val);
        return !contained;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        boolean contained = locs.containsKey(val);
        if (!contained) {
            return false;
        }
        int index = locs.get(val).iterator().next();
        locs.get(val).remove(index);
        if (index < nums.size()-1) {
            int change = nums.get(nums.size()-1);
            nums.set(index, change);
            locs.get(change).remove(nums.size()-1);
            locs.get(change).add(index);
        }
        nums.remove(nums.size()-1);
        if (locs.get(val).isEmpty()) {
            locs.remove(val);
        }
        return true;
        
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }
}

/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * RandomizedCollection obj = new RandomizedCollection();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */