Leetcode 380. Insert Delete GetRandom O(1)

Question:

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

Explanation:

很神奇的一道题，因为需要每个操作的复杂度平均为O(1),最初我只用了一个Arraylist存也AC了，但是时间很久，因为remove的时候不是O(1).换了Hashmap存index，快一些。但是要注意，remove那边不能直接count--，需要和最后一个值调换，这样保证其他的index不变，只减少最后一个。A little tricky 总而言之，算法设计还是蛮有趣的，好好想想。

Code:

public class RandomizedSet {

    /** Initialize your data structure here. */
    List<Integer> list = new ArrayList<>();
    HashMap<Integer, Integer> map = new HashMap<>();
    int count;
    public RandomizedSet() {
        count = 0;
    }

    /** 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,count);
        list.add(val);
        count=map.size();
        // System.out.println(list);
        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 index = map.get(val);
            // change the index with the last one if it is not the last one;
            if (index < list.size()-1) {
                int lastone = list.get(list.size()-1);
                list.set(index, lastone);
                map.put(lastone,index);
            }
            map.remove(val);
            list.remove(list.size()-1);
            return true;
        }

        return false;

    }

    /** Get a random element from the set. */
    public int getRandom() {
        int r =(int)(Math.random()*list.size());
        // System.out.println(r);
        return list.get(r);
    }
}

/**
 * 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();
 */