Insert Delete GetRandom O(1) - Duplicates allowed in JAVA

题目描述：
Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.
insert(val): Inserts an item val to the collection.
remove(val): Removes an item val from the collection if present.
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();

简单说就是构造一个数据结构，能够插入，删除以及getrandom。允许重复的值，但都是integer。

难处就在于有O(1)复杂度的限制，因此用HashMap/HashSet.
note: hashMap 的平均复杂度是O(1),记住就好

所以解题思路大致是：一个HashMap(Integer, HashSet(Integer)) h1用来保存 value

一个HashMap(Integer, HashSet(Integer)) h1用来保存 value以及其对应的key值的set，再用另一 HashMap(Integer,Integer) h2来保存key值与value的mapping关系。

insert的想法很简单，插入一个的时候就在h1 中value对应的set中添加key值， h2中直接一个一个添加key-value对。

remove的想法是：h1中从val对应的key set中任意选一个key删除， h2中将此key对应的value与h2末尾的value调换，并对应更新h1，删除h2末尾，删除h1相应的set中的key。
我觉得h2这么做的原因是为了getrandom时候可以直接random*h2.size()得到randomly，减少复杂度

import java.util.HashMap;import java.util.HashSet;import java.util.Iterator;public class RandomizedCollection {    HashMap<Integer,Integer> h2;    HashMap<Integer,HashSet<Integer>>h1;     public RandomizedCollection() {            h1=new HashMap<Integer,HashSet<Integer>>();            h2=new HashMap<Integer,Integer>();        }        /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */        public boolean insert(int val) {            boolean contain=true;//          System.out.println(h1.size()+",,,"+h2.size());            if(h1.containsKey(val)){                HashSet s=h1.get(val);                s.add(h2.size());                contain=false;}            else{ contain=true;            HashSet s=new HashSet();            s.add(h2.size());            h1.put(val, s);            }            h2.put(h2.size(), val);            return contain;        }        /** Removes a value from the collection. Returns true if the collection contained the specified element. */        public boolean remove(int val) {            boolean contain=false;            if(h1.containsKey(val)){                contain=true;        Iterator<Integer>   keyIter=h1.get(val).iterator();        int key=keyIter.next();//      System.out.println("start Val:"+val);        HashSet<Integer>s=h1.get(val);//      System.out.println("start h2(key)"+h2.get(key));       if (s.size()-1==0){//         560503203817//         System.out.println("val:"+val);           int lastVal=h2.get(h2.size()-1);//         System.out.println("lastVal:"+lastVal);//         System.out.println(val!=lastVal);          if(val!=lastVal){             HashSet<Integer> s2=h1.get(lastVal);             s2.remove(h2.size()-1);             s2.add(key);           h2.put(key, lastVal);}          h1.remove(val);           h2.remove(h2.size()-1);           }       else {           if(key==h2.size()-1){               s.remove(key);//                     h2.remove(h2.size()-1);}      else{          int lastVal=h2.get(h2.size()-1);          if(val==lastVal){              s.remove(h2.size()-1);              h2.remove(h2.size()-1);          }          else{              s.remove(key);          HashSet<Integer>s2=h1.get(lastVal);          s2.add(key);          s2.remove(h2.size()-1);        h2.put(key, lastVal);          h2.remove(h2.size()-1);}      }}            }            return contain;        }        /** Get a random element from the collection. */        public int getRandom() {            if (h2.size()==0)return 0;            int rand=(int)(Math.random()*(h2.size()));       //   System.out.println(rand);            return h2.get(rand);        }}