Leetcode: Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.
Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

最先想到是用double LinkedList+Map, 没必要，arraylist+map就够了；另外取random的方法还有，rand.nextInt(int n) returns an integer in the range [0, n)

java.util.Random rand = new java.util.Random();


return nums.get( rand.nextInt(nums.size()) );

 public class RandomizedSet {
     HashMap<Integer, Integer> map;
     ArrayList<Integer> arr;

     /** Initialize your data structure here. */
     public RandomizedSet() {
         map = new HashMap<Integer, Integer>();
         arr = new ArrayList<Integer>();
     }

     /** 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;
         arr.add(val);
         map.put(val, arr.size()-1);
         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 lastItem = arr.get(arr.size()-1);
         if (val != lastItem) {
             int index = map.get(val);
             arr.set(index, lastItem);
             map.put(lastItem, index);
         }
         arr.remove(arr.size()-1);
         map.remove(val);
         return true;
     }

     /** Get a random element from the set. */
     public int getRandom() {
         return arr.get((int)(Math.random()*arr.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();
  */