380. Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

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();

My Solutions:

题目要求在O(1)的时间内完成插入, 删除, 取随机元素。

• 用linked lilst保存元素可以在O(1)内完成插入和删除, 但是无法在O(1)随机元素。

• 用数组可以在O(1)内完成插入和随机, 但是删除无法在O(1)完成。

如果用另外一种删除方式, 即把要删除的元素与最后的元素交换位置, 而删除最后一个元素就可以在O(1)完成。这样的话需要一个hash表来记录元素在数组的位置, 这样才可以O(1)取到这个元素, 并移动到尾部。还需要注意的是在删除一个元素的时候与另外一个元素交换了位置, 所以在hash表中还需要将另外一个元素的位置更新。由于哈希表的新增/删除操作是O(1)，而数组的随机访问操作开销也是O(1)，因此满足题设要求

用hashmap和list同时储存value和index。Use hashmap<int, int> set for insert and getRandom to store value and index; Use list的自带index位置储存value，list.get(index)可以达到O(1) 的insert和getRandom

难点是如何让remove也是O(1) - Use list<int> for remove[O(1)]: Always add the element to the end of the list, so when you need to remove a value, swap last element with the want-to-delete element:

1. find the last elem;

2. set last element to be at index position;

3. put last element in index position

class RandomizedSet {

    HashMap<Integer, Integer> set;
    List<Integer> list;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        set = 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 (set.containsKey(val)) return false;
        set.put(val, set.size()); //insert到最后一个位置
        list.add(val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!set.containsKey(val)) return false;
        int index = set.get(val);
        
        //swap last element with the want-to-delete element
        if (index != list.size() - 1) {
            int last = list.get(list.size() - 1); //last element
            list.set(index, last); // set last element to be at index position in list
            set.put(last, index); // put last element in index position
        }
        set.remove(val);
        list.remove(list.size() - 1);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Random r = new Random();
        int index = r.nextInt(set.size());
        return list.get(index);
    }
}

/**
 * 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();
 */