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); } }