[leetcode]381. Insert Delete GetRandom O(1) - Duplicates allowed-优快云博客

本文介绍了一个LeetCode上的数据结构题目的解决方案——实现一个允许重复元素的随机集合。该集合支持平均时间复杂度为O(1)的插入、删除及随机返回集合中元素的操作。

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题目链接：https://leetcode.com/problems/insert-delete-getrandom-o1-duplicates-allowed/description/

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();

There are two data member in the solution

a vector nums
an unordered_map m

The key of m is the val, the value of m is a vector contains indices where the val appears in nums.
Each element of nums is a pair, the first element of the pair is val itself, the second element of the pair is an index into m[val].
There is a relationship between nums and m:

m[nums[i].first][nums[i].second] == i;

class RandomizedCollection {
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) {
        auto result = m.find(val) == m.end();
        
        m[val].push_back(nums.size());
        nums.push_back(pair<int, int>(val, m[val].size() - 1));
        
        return result;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
        auto result = m.find(val) != m.end();
        if(result)
        {
            auto last = nums.back();
            m[last.first][last.second] = m[val].back();
            nums[m[val].back()] = last;
            m[val].pop_back();
            if(m[val].empty()) m.erase(val);
            nums.pop_back();
        }
        return result;
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
        return nums[rand() % nums.size()].first;
    }
private:
    vector<pair<int, int>> nums;
    unordered_map<int, vector<int>> m;
};