Is there a way to get the value of a HashMap randomly in Java?

Question

Is there a way to get the value of a HashMap randomly in Java?

Answer 1

If you are fine with O(n) time complexity you can use methods like values() or values().toArray() but if you look for a constant O(1) getRandom() operation one great alternative is to use a custom data structure. ArrayList and HashMap can be combined to attain O(1) time for insert(), remove() and getRandom(). Here is an example implementation:

class RandomizedSet {
    List nums = new ArrayList<>();
    Map valToIdx = new HashMap<>();
    Random rand = new Random();

    public RandomizedSet() { }

    /**
     * Inserts a value to the set. Returns true if the set did not already contain
     * the specified element.
     */
    public boolean insert(int val) {
        if (!valToIdx.containsKey(val)) {
            valToIdx.put(val, nums.size());
            nums.add(val);
            return true;
        }
        return false;
    }

    /**
     * Removes a value from the set. Returns true if the set contained the specified
     * element.
     */
    public boolean remove(int val) {
        if (valToIdx.containsKey(val)) {
            int idx = valToIdx.get(val);
            int lastVal = nums.get(nums.size() - 1);

            nums.set(idx, lastVal);
            valToIdx.put(lastVal, idx);

            nums.remove(nums.size() - 1);
            valToIdx.remove(val);
            return true;
        }
        return false;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }
}

The idea comes from this problem from leetcode.com.