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);
}

}