151.Insert Delete GetRandom O(1) - Duplicates allowed

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

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned islinearly related to the number of same value the collection contains.

Example:

// Init an empty collection.RandomizedCollection collection = new RandomizedCollection();// Inserts 1 to the collection. Returns true as the collection did not contain 1.collection.insert(1);// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].collection.insert(1);// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].collection.insert(2);// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.collection.getRandom();// Removes 1 from the collection, returns true. Collection now contains [1,2].collection.remove(1);// getRandom should return 1 and 2 both equally likely.collection.getRandom();

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分析，与前一道题目类似，不一样的地方在于，集合中可以存在重复的元素，但是在插入的时候如果已存在需要插入的val时，则返回false，但是还是要把val插入到集合中。

 ArrayList<Integer> list;  /** Initialize your data structure here. */    public RandomizedCollection() {    list = new ArrayList<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 flag = !list.contains(val);        list.add(val);        return flag;    }        /** Removes a value from the set. Returns true if the set contained the specified element. */    public boolean remove(int val) {    if(!list.contains(val)){    return false;    }else{    list.remove((Integer)val);    return true;    }    }        /** Get a random element from the set. */    public int getRandom() {    if (list.size() == 1) {// 如果全排列的个数小于等于1个，则只需要返回base数组即可。return  list.get(0);}/* 随机生成一个下标 */Random random = new Random();int i = random.nextInt(list.size());return list.get(i);    }