Massive Algorithms: 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.

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();

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list即可

private:

 unordered_map<int, vector<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].push_back(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 _id = index[val].back(); index[val].pop_back();

  int last = output.back(); output.pop_back();

  if (last != val) {

   index[last].back() = _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()];

}

https://zxi.mytechroad.com/blog/hashtable/leetcode-381-insert-delete-getrandom-o1-duplicates-allowed/

https://leetcode.com/problems/insert-delete-getrandom-o1-duplicates-allowed/discuss/85540/Java-HaspMap-LinkedHashSet-ArrayList-(155-ms)

public class RandomizedCollection {
    ArrayList<Integer> nums;
 HashMap<Integer, Set<Integer>> locs;
 java.util.Random rand = new java.util.Random();
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        nums = new ArrayList<Integer>();
     locs = new HashMap<Integer, Set<Integer>>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean contain = locs.containsKey(val);
     if ( ! contain ) locs.put( val, new LinkedHashSet<Integer>() ); 
     locs.get(val).add(nums.size());        
     nums.add(val);
     return ! contain ;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        boolean contain = locs.containsKey(val);
     if ( ! contain ) return false;
     int loc = locs.get(val).iterator().next();
     locs.get(val).remove(loc);
     if (loc < nums.size() - 1 ) {
        int lastone = nums.get( nums.size()-1 );
        nums.set( loc , lastone );
        locs.get(lastone).remove( nums.size()-1);
        locs.get(lastone).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( rand.nextInt(nums.size()) );
    }
}

https://github.com/mintycc/OnlineJudge-Solutions/blob/master/Leetcode/381_Insert_Delete_GetRandom_O(1)_Duplicates_Allowed.java

LeetCode 381 - Insert Delete GetRandom O(1) - Duplicates allowed

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