381. Insert Delete GetRandom O(1) Duplicates allowed - jiejackyzhang/leetcode-note GitHub Wiki

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();

380题的follow up。 为了存储duplicate number，可以使用set来存储duplicates的位置。 这里使用LinkedHashSet，因为"Using LinkedHashSet can be considered as O(1) if we only get the first element to remove"。

这里在remove时，应先将val的位置set中的loc先删除，再与最后元素交换如果不是最后一个的话。 这是由于duplicates的存在，若交换的元素与删除的元素是duplicates的话，在之后删除loc会把交换之后的元素也删掉了。

public class RandomizedCollection {
    private List<Integer> nums;
    private Map<Integer, Set<Integer>> locs;
    private Random random;

    /** Initialize your data structure here. */
    public RandomizedCollection() {
        nums = new ArrayList<>();
        locs = new HashMap<>();
        random = 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 isContain = locs.containsKey(val);
        if(!isContain) {
            locs.put(val, new LinkedHashSet<>());
        }
        locs.get(val).add(nums.size());
        nums.add(val);
        return !isContain;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if(!locs.containsKey(val)) return false;
        int loc = locs.get(val).iterator().next();
        locs.get(val).remove(loc);
        if(loc < nums.size() - 1) {
            int lastnode = nums.get(nums.size()-1);
            nums.set(loc, lastnode);
            locs.get(lastnode).remove(nums.size()-1);
            locs.get(lastnode).add(loc);
        }
        nums.remove(nums.size()-1);
        if(locs.get(val).isEmpty()) locs.remove(val);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(random.nextInt(nums.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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