381. Insert Delete GetRandom O(1) - Duplicates allowed

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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();

解法1：

自己写的怎么都没过OA,这个AC的答案是网上抄的[捂脸]。主要的思想和没有duplicates的版本很相似。区别就是这里对于每一个数字，要存储的是出现的index的list。删除操作的时候，把末尾的元素直接写到被删除的元素的位置。同时维护一下两个index的list.
对于需要删除的数字，去掉list中相应的位置
对于末尾的数字，在list中加入要删除的数字的位置
在队列中把要删除的元素的位置置换为末尾的元素
删除队列末尾的元素（因为已经放到了队列中间去了)
C++

1

Java

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public class RandomizedCollection {
    ArrayList<Integer> result;
    HashMap<Integer, LinkedHashSet<Integer>> map;
    public RandomizedCollection() {
        result = new ArrayList<Integer>();
        map = new HashMap<Integer, LinkedHashSet<Integer>>();
    }
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        // Add item to map if it doesn't already exist.
        boolean alreadyExists = map.containsKey(val);
        if(!alreadyExists) {
            map.put(val, new LinkedHashSet<Integer>());
        }
        map.get(val).add(result.size());
        result.add(val);
        return !alreadyExists;
    }
    /** 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;
        }
        // Get arbitary index of the ArrayList that contains val
        LinkedHashSet<Integer> valSet = map.get(val);
        int indexToReplace = valSet.iterator().next();
        // Obtain the set of the number in the last place of the ArrayList
        int numAtLastPlace = result.get(result.size() - 1);
        LinkedHashSet<Integer> replaceWith = map.get(numAtLastPlace);
        // Replace val at arbitary index with very last number
        result.set(indexToReplace, numAtLastPlace);
        // Remove appropriate index
        valSet.remove(indexToReplace);
        // Don't change set if we were replacing the removed item with the same number
        if(indexToReplace != result.size() - 1) {
            replaceWith.remove(result.size() - 1);
            replaceWith.add(indexToReplace);
        }
        result.remove(result.size() - 1);
        // Remove map entry if set is now empty, then return
        if(valSet.isEmpty()) {
            map.remove(val);
        }
        return true;
    }
    /** Get a random element from the collection. */
    public int getRandom() {
        // Get linearly random item
        Random random = new Random();
        return result.get(random.nextInt(result.size()));
        // return result.get((int)(Math.random() * result.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();
 */