[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();

380. Insert Delete GetRandom O(1)的拓展，这题是可以有重复数字。只需将上一题目的解法稍作改动，依然使用哈希表+数组，这次哈希表中的值是保存数组下标的一个Set。

Java：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Random;
import java.util.TreeSet;

public class RandomizedCollection {

    private HashMap<Integer, TreeSet<Integer>> dataMap;
    private ArrayList<Integer> dataList;
     /** Initialize your data structure here. */
    public RandomizedCollection() {
        dataMap = new HashMap<Integer, TreeSet<Integer>>();
        dataList = new ArrayList<Integer>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        TreeSet<Integer> idxSet = dataMap.get(val);
        if (idxSet == null) {
            idxSet = new TreeSet<Integer>();
            dataMap.put(val, idxSet);
        }
        idxSet.add(dataList.size());
        dataList.add(val);
        return idxSet.size() == 1;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        TreeSet<Integer> idxSet = dataMap.get(val);
        if (idxSet == null || idxSet.isEmpty()) {
            return false;
        }
        int idx = idxSet.pollLast(); //Last index of val
        int tail = dataList.get(dataList.size() - 1); //Tail of list
        TreeSet<Integer> tailIdxSet = dataMap.get(tail);
        if (tail != val) {
            tailIdxSet.pollLast(); //Remove last idx of list tail
            tailIdxSet.add(idx); //Add idx to tail idx set
            dataList.set(idx, tail);
        }
        dataList.remove(dataList.size() - 1);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return dataList.get(new Random().nextInt(dataList.size()));
    }
}

Python:

from random import randint
from collections import defaultdict

class RandomizedCollection(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.__list = []
        self.__used = defaultdict(list)
        

    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
        """
        has = val in self.__used

        self.__list += val,
        self.__used[val] += len(self.__list)-1,

        return not has


    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.__used:
            return False

        self.__used[self.__list[-1]][-1] = self.__used[val][-1]
        self.__list[self.__used[val][-1]], self.__list[-1] = self.__list[-1], self.__list[self.__used[val][-1]]

        self.__used[val].pop()
        if not self.__used[val]:
            self.__used.pop(val)
        self.__list.pop()

        return True

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

C++:

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) {
        m[val].insert(nums.size());
        nums.push_back(val);
        return m[val].size() == 1;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
        if (m[val].empty()) return false;
        int idx = *m[val].begin();
        m[val].erase(idx);
        if (nums.size() - 1 != idx) {
            int t = nums.back();
            nums[idx] = t;
            m[t].erase(nums.size() - 1);
            m[t].insert(idx);
        } 
        nums.pop_back();
        return true;
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
        return nums[rand() % nums.size()];
    }

private:
    vector<int> nums;
    unordered_map<int, unordered_set<int>> m;
};

　　

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