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.

// 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();

这道题刚开始做的时候大意了，没有太明白题目意思，常用数据结构中基本都能完成常量时间内添加元素功能，但是在常量时间内删除元素功能例如数组就不能完成，因此需要诸如Map、Set这类可以在常量时间内定位到元素的数据结构，另外一个要求是需要返回随机元素，这时就需要我们的元素有一定顺序，因此需要类似于数组之类的数据结构，最后选用HashMap和ArrayList完成该数据结构的创建，代码如下：

ArrayList<Integer> arrayList = null;    HashMap<Integer,Integer> hashMap = null;        /** Initialize your data structure here. */    public RandomizedSet() {        arrayList = new ArrayList<Integer>();        hashMap = new HashMap<Integer, Integer>();    }        /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */    public boolean insert(int val) {        boolean result = hashMap.containsKey(val);        if(result){            return false;        }else{            hashMap.put(val, arrayList.size());            arrayList.add(val);            return true;        }    }        /** Removes a value from the set. Returns true if the set contained the specified element. */    public boolean remove(int val) {        boolean result = hashMap.containsKey(val);        int pos = 0;        if(result){            pos = hashMap.get(val);            arrayList.set(pos, arrayList.get(arrayList.size() - 1));            hashMap.put(arrayList.get(pos), pos);        }else{            return false;        }        hashMap.remove(val);        arrayList.remove(arrayList.size() - 1);        return true;    }        /** Get a random element from the set. */    public int getRandom() {        // Random rand = new Random();        // return arrayList.get(rand.nextInt(arrayList.size()));        int num = (int)(Math.random() * arrayList.size());        return arrayList.get(num);    }

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