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

Arnab Chakraborty

Published on 01-Jun-2020 10:44:08