[LeetCode] 381. Insert Delete GetRandom O(1)

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();

O(1) 时间插入、删除和获取随机元素 - 允许重复。题意跟380题很接近，唯一不同的条件是这个题允许加入相同元素。

思路还是用hashmap + arrayList，hashmap的key是每个不同的数字，value是一个hashset，存的是每个数字在list里的index。

insert() - 遇到相同的数字的时候，每次记得先在hashmap的key中加入list当前的size再处理list的部分。arrayList还是保存每一个被加入的num。

remove() - 从hashmap里删除的时候，如果某个元素出现过多次，则删除一次就好，此时需要对hashset做iterate；如果这个元素只出现了一次，记得要将hashmap中的key也一并删除。对list做删除操作的时候，跟380题一样，也是需要把list的最后一个元素移动到list里被删除的元素的位置上。

时间O(1) - required

空间O(n)

Java实现

class RandomizedCollection {
    HashMap<Integer, HashSet<Integer>> map;
    List<Integer> list;
    Random rmd;

    /** Initialize your data structure here. */
    public RandomizedCollection() {
        map = new HashMap<>();
        list = new ArrayList<>();
        rmd = new Random();
    }
    
    /** 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 = map.containsKey(val);
        if (!contain) {
            map.put(val, new HashSet<>());
        }
        map.get(val).add(list.size());
        list.add(val);
        return !contain;
    }
    
    /** 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;
        }
        int firstIndex = map.get(val).iterator().next();
        map.get(val).remove(firstIndex);
        if (map.get(val).size() == 0) {
            map.remove(val);
        }
        int lastVal = list.remove(list.size() - 1);
        if (firstIndex != list.size()) {
            list.set(firstIndex, lastVal);
            map.get(lastVal).remove(list.size());
            map.get(lastVal).add(firstIndex);
        }
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(rmd.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();
 */

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