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

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

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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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

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insert, delete 用hashmap操作，get random用arraylist，map存<val, idx>, idx對應(yīng)val在list中的索引

insert: 如果hash map中存在val，返回false；如果不存在，先在list末尾添加該元素，再把val及其在list中的坐標(biāo)加入map中

delete: 當(dāng)要刪除元素不是list的末尾元素，為了達(dá)到O(1)操作，先把要刪除的元素和最后一個(gè)元素交換（把要刪除元素索引處的值設(shè)為最后一個(gè)元素的值，并在map中放入該元素在list中的新坐標(biāo)），再刪除list最后一個(gè)元素

getRandom: 直接返回list中隨機(jī)元素即可

class RandomizedSet {
    Map<Integer, Integer> map;
    List<Integer> list;
    Random r = new Random();

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
    }
    
    /** 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;
        }
        list.add(val);
        map.put(val, list.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 idx = map.get(val);
        if(idx < list.size() - 1) {
            int last = list.get(list.size() - 1);
            list.set(idx, last);
            map.put(last, idx);
        }
        map.remove(val);
        list.remove(list.size() - 1);
        
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(r.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();
 */

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