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.

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();

思路：

那么查询还是直接操作ArrayList没有难度

而插入的时候，是插入到位置集合也没有难度

删除的时候，选择Stack，需要保证出栈的顺序有保障

class RandomizedCollection {

//用来索引下在ArrayList中的关系，做到快速定位
private HashMap<Integer, Stack<Integer>> locations;
//存储原始数据
private ArrayList<Integer> list;
private Random random;
//为了排序，控制栈的
ArrayList<Integer> tmpForStack;
/** Initialize your data structure here. */
public RandomizedCollection() {
this.locations = new HashMap<Integer, Stack<Integer>>();
this.list = new ArrayList<Integer>();
this.random = new Random();
this.tmpForStack = 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) {
boolean flag = false;
if(locations.containsKey(val) == false){
locations.put(val, new Stack<Integer>());
}
if(locations.get(val).isEmpty())
flag = true;
locations.get(val).push(list.size());
list.add(val);
return flag;
}

/** Removes a value from the collection. Returns true if the collection contained the specified element. */
public boolean remove(int val) {
if(locations.containsKey(val) == false || locations.get(val).isEmpty())
return false;
int tmpLocation = locations.get(val).pop();
if(tmpLocation != list.size() - 1){
int lastVal = list.get(list.size() -1);
list.set(tmpLocation, lastVal);
locations.get(lastVal).pop();
tmpForStack.clear();
while(locations.get(lastVal).isEmpty() == false && locations.get(lastVal).peek() > tmpLocation){
tmpForStack.add(locations.get(lastVal).pop());
}
locations.get(lastVal).push(tmpLocation);
for(int tval:tmpForStack)
locations.get(lastVal).push(tval);
}
list.remove(list.size() - 1);
return true;
}

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