LeetCode 381. Insert Delete GetRandom O(1)

原题链接在这里：https://leetcode.com/problems/insert-delete-getrandom-o1-duplicates-allowed/

题目：

Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. 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();

题解：

用set来保存val的所有index.

Time Complexity: insert, O(1). remove, O(1). getRandom, O(1).

Space: O(n).

AC Java:

class RandomizedCollection {
    ArrayList<Integer> list;
    HashMap<Integer, LinkedHashSet<Integer>> numToInds;
    
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        list = new ArrayList<>();
        numToInds = new HashMap<>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean contains = numToInds.containsKey(val);
        if(!contains){
            numToInds.put(val, new LinkedHashSet<Integer>());
        }
        
        numToInds.get(val).add(list.size());
        list.add(val);
        return contains;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if(!numToInds.containsKey(val) || numToInds.get(val).size() == 0){
            return false;
        }
        
        LinkedHashSet<Integer> lhs = numToInds.get(val);
        int ind = lhs.iterator().next();
        lhs.remove(ind);
        int last = list.get(list.size() - 1);
        list.set(ind, last);
        numToInds.get(last).add(ind);
        numToInds.get(last).remove(list.size() - 1);
        list.remove(list.size() - 1);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        Random rand = new Random();
        return list.get(rand.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();
 */

类似Insert Delete GetRandom O(1).