[LeetCode] 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.

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();

因为需要o(1) 时间复杂度实现getRandom() 所以需要ArrayList 另外insert和remove也要是o(1)，所以需要HashMap
代码如下:
public class RandomizedCollection {
//首先 ArrayList删除最后一个元素的时间复杂度是o(1) 具体参见jdk源码
ArrayList<Integer> list;
HashMap<Integer,ArrayList<Integer>> map;
Random ran=new Random();

/** Initialize your data structure here. */
public RandomizedCollection() {
list=new ArrayList<Integer>();
map=new HashMap<Integer,ArrayList<Integer>>();
}

/** Inserts a value to the collection. Returns true if the collection did not already contain the specified el
4000
ement. */
public boolean insert(int val) {
list.add(val);
if(!map.containsKey(val)){
map.put(val,new ArrayList<Integer>());
}
map.get(val).add(list.size()-1);
return true;
}

/** Removes a value from the collection. Returns true if the collection contained the specified element. */
public boolean remove(int val) {
if(!map.containsKey(val)) return false;
ArrayList<Integer> mapList=map.get(val);
int pos=mapList.remove(mapList.size()-1);
if(mapList.isEmpty()) map.remove(val);
if(pos!=list.size()-1){
Integer temp=list.get(list.size()-1);
list.set(pos,temp);
map.get(temp).remove(new Integer(list.size()-1));
map.get(temp).add(pos);
}
list.remove(list.size()-1);
return true;
}

/** Get a random element from the collection. */
public int getRandom() {
return list.get(ran.nextInt(list.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();
*/

之前写的代码不不符合条件的，getRandom不是o(1)。
public class RandomizedCollection {
Random random=new Random();
HashMap<Integer,Integer> map;
int size;
/** Initialize your data structure here. */
public RandomizedCollection() {
map=new HashMap<Integer,Integer>();
size=0;
}

/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
public boolean insert(int val) {
size++;
if(!map.containsKey(val)){
map.put(val, 1);
return true;
}else{
map.put(val, map.get(val)+1);
return false;
}
}
/** Removes a value from the collection. Returns true if the collection contained the specified element. */
public boolean remove(int val) {
if(!map.containsKey(val)) return false;
else{
Integer a=map.get(val);
if(a<=1) map.remove(val);
else{
map.put(val, a-1);
}
size--;
}
return true;

}
/** Get a random element from the collection. */
public int getRandom() {
int ran=random.nextInt(size);
int i=-1;
for(Integer a:map.keySet()){
int times=map.get(a);
if(ran>i&&ran<=i+times){
return a;
}
i+=times;
}
return -1;
}
}

/**
* 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();
*/