题目描述：

LeetCode 381. Insert Delete GetRandom O(1) - Duplicates allowed

Design a data structure that supports all following operations in average 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 is linearly 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();

题目大意：

设计一个数据结构支持在O(1)时间内完成如下操作：

注意：允许重复元素。

insert(val): 如果集合中不包含val，则插入val

remove(val): 如果集合中包含val，则移除val

getRandom: 从现有集合中随机返回一个元素，每个元素被返回的概率应该与其在集合中的数量线性相关。

解题思路：

本题是LeetCode 380. Insert Delete GetRandom O(1)的升级版，新增了允许重复元素的限制条件。

只需将上述题目的解法稍作改动，依然使用哈希表+数组的组合，只不过哈希表中的值保存数组下标的Set即可。

Java代码：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Random;
import java.util.TreeSet;

public class RandomizedCollection {

    private HashMap<Integer, TreeSet<Integer>> dataMap;
    private ArrayList<Integer> dataList;
     /** Initialize your data structure here. */
    public RandomizedCollection() {
        dataMap = new HashMap<Integer, TreeSet<Integer>>();
        dataList = new ArrayList<Integer>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        TreeSet<Integer> idxSet = dataMap.get(val);
        if (idxSet == null) {
            idxSet = new TreeSet<Integer>();
            dataMap.put(val, idxSet);
        }
        idxSet.add(dataList.size());
        dataList.add(val);
        return idxSet.size() == 1;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        TreeSet<Integer> idxSet = dataMap.get(val);
        if (idxSet == null || idxSet.isEmpty()) {
            return false;
        }
        int idx = idxSet.pollLast(); //Last index of val
        int tail = dataList.get(dataList.size() - 1); //Tail of list
        TreeSet<Integer> tailIdxSet = dataMap.get(tail);
        if (tail != val) {
            tailIdxSet.pollLast(); //Remove last idx of list tail
            tailIdxSet.add(idx); //Add idx to tail idx set
            dataList.set(idx, tail);
        }
        dataList.remove(dataList.size() - 1);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return dataList.get(new Random().nextInt(dataList.size()));
    }
}

/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * RandomizedCollection obj = new RandomizedCollection();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */

 

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