leetcode Insert Delete GetRandom O(1) - Duplicates allowed

leetcode 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.

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();

题目地址：leetcode Insert Delete GetRandom O(1) - Duplicates allowed

题目大意: 要求实现一个数据结构（数允许重复），可以支持插入，删除，随机数生成。 他们的复杂度均要求O(1)

思路：其实和上一题  leetcode Insert Delete GetRandom O(1) 差不多，只不过这题允许重复，把字典里换成一个哈希表的set即可(注意，不可以是list)

比如如下的数据，list无法保证顺序！特别感谢@guanglightman 指出

["RandomizedCollection", "insert", "insert", "insert", "insert", "insert", "insert", "insert", "insert","remove", "remove", "remove", "remove", "remove", "remove"]
[[], [3], [4], [5], [6], [1], [2], [1], [2], [3], [4], [5], [6], [2], [2]]

 

C++

class RandomizedCollection {
private:
	unordered_map<int, unordered_set<int>> index;
	vector<int> output;
public:
	/** Initialize your data structure here. */
	RandomizedCollection() {}

	/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
	bool insert(int val) {
		bool return_val = index.find(val) == index.end();
		index[val].insert(output.size());
		output.push_back(val);
		return return_val;
	}

	/** Removes a value from the collection. Returns true if the collection contained the specified element. */
	bool remove(int val) {
		if (index.find(val) == index.end()) return false;
		int last = output.back(); output.pop_back();
		index[last].erase(output.size());
		if (last != val) {
			int _id = *index[val].begin(); index[val].erase(_id);
			index[last].insert(_id);
			output[_id] = last;
		}
		if (index[val].empty())
			index.erase(val);
		return true;
	}

	/** Get a random element from the collection. */
	int getRandom() {
		return output[rand() % output.size()];
	}
};

 

Python

import random

class RandomizedCollection(object):
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.index = collections.defaultdict(set)
        self.output = []

    def insert(self, val):
        """
        Inserts a value to the collection. Returns true if the collection did not already contain the specified element.
        :type val: int
        :rtype: bool
        """
        return_val = val not in self.index
        self.index[val].add(len(self.output))
        self.output.append(val)
        return return_val

    def remove(self, val):
        """
        Removes a value from the collection. Returns true if the collection contained the specified element.
        :type val: int
        :rtype: bool
        """
        if val not in self.index: return False
        last = self.output.pop()
        self.index[last].remove(len(self.output))
        if val != last:
            index = self.index[val].pop()
            self.index[last].add(index)
            self.output[index] = last
        if not self.index[val]:
            del self.index[val]
        return True

    def getRandom(self):
        """
        Get a random element from the collection.
        :rtype: int
        """
        return self.output[random.randint(0, len(self.output) - 1)]

 

本题是leetcode 381 Insert Delete GetRandom O(1) - Duplicates allowed 的题解

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