380. Insert Delete GetRandom O(1)

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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();

设计一个符合要求的数据结构；

设计题的答案比较好理解，但是不容易想到。

这个题要求O(1) 时间复杂度不知道能不能符合要求。

利用hashmap和list的配合来做。hash可以判断值是否存在，list可以设置给定位置的值。

保证从0-list.size()的索引内都有值一一对应

public class RandomizedSet {    HashMap<Integer,Integer> map;    List<Integer> list;    /** 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;        map.put(val,list.size());        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(!map.containsKey(val)) return false;        int value=map.get(val);        int lastEle=list.get(list.size()-1);        list.set(value,lastEle);        list.remove(list.size()-1);        map.put(lastEle,value);        map.remove(val);        return true;    }        /** Get a random element from the set. */    public int getRandom() {        Random ran=new Random();        int a=ran.nextInt(list.size());        return list.get(a);    }}/** * 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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