Insert Delete GetRandom O(1) - Duplicates allowed in C++

Suppose, we want to make a data structure, that supports some operations, these operations must be preformed in O(1) amount of time. So let these operations are like −

• insert(x): insert x into the collection
• remove(x): delete x from the collection
• getRandom(): This will find random element form that collection.

To solve this, we will follow these steps −

• Make an array nums
• make one map m
• Define a function insert(), this will take val,
• ret := when val is not in m
• insert size of nums at the end of m[val]
• insert { val, size of m[val] – 1} pair at the end of nums
• return ret
• Define a function remove(), this will take val,
• ret := when val is not in m
• if ret is non-zero, then −
  • last = last element of nums
  • m[key of last][value of last] := last element of m[val]
  • nums[last element of [m[val]] := last
  • delete last element from m[val]
  • if m[val] is empty, then −
    • delete val from m
  • delete last element from nums
• return ret
• Define a function getRandom()
• return one random element from the collection

Let us see the following implementation to get better understanding −

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class RandomizedCollection {
public:
   vector <pair <int, int>> nums;
   unordered_map <int, vector<int>> m;
   RandomizedCollection() {
   }
   bool insert(int val) {
      bool ret = m.find(val) == m.end();
      m[val].push_back(nums.size());
      nums.push_back({val, m[val].size() - 1});
      return ret;
   }
   bool remove(int val) {
      bool ret = m.find(val) != m.end();
      if(ret){
         pair <int, int> 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 ret;
   }
   int getRandom() {
      return nums[rand() % nums.size()].first;
   }
};
main(){
   RandomizedCollection ob;
   ob.insert(10);
   ob.insert(35);
   ob.insert(20);
   ob.insert(40);
   cout << (ob.getRandom()) << endl;
   ob.remove(20);
   cout << (ob.getRandom()) << endl;
}

Input

Insert 10, 35, 20, 40, then get one random number, say 40, then remove 20, again get random element, that is say 35.

Output

40
35