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

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.

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();

There are two data member in the solution

1. a vector nums
2. 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;};