Leetcode/各大家-- 380. Insert Delete GetRandom O(1) (Random + Design)

380. Insert Delete GetRandom O(1)
medium
https://leetcode.com/problems/insert-delete-getrandom-o1/

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

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.

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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思路：Use HashMap + ArrayList to build the data structure.
          Map store pairs of <val, index in list> and List is store the val existed.
          They are same size. Add and remove together
          When removal, list: replace the removed element with last added element.
                                    map: replace the index(val) of last added element, and remove val(key)

Complexity: Space: O(N)

ublic class RandomizedSet {
    Map<Integer, Integer> map;//<val, index in arrlist>
    List<Integer> list;//keep a record of value for get random 
    int size;//map and list are the same size
    Random rand;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.map = new HashMap<>();
        this.list = new ArrayList<>();
        this.size = 0;
        this.rand = new Random();
    }
    
    /** 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;
        map.put(val, size);
        list.add(val);
        size++;
        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)){
            int last = list.get(size - 1);
            list.set(map.get(val), last);//use last added ele to fill the element to be removed 
            map.put(last, map.get(val));//update last added ele's index in list
            map.remove(val);
            list.remove(size - 1);//remove at end is constant
            size--;
            return true;
        }
        return false;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rand.nextInt(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();
 */