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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
#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
Updated on: 01-Jun-2020
65 Views
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