First Unique Number in C++

Suppose we have a queue of integers, we need to retrieve the first unique integer in that queue. We have to implement the class called FirstUnique: It will be initialized by the numbers in the queue. Define one function showFirstUnique(), this will return the value of the first unique integer of the queue and returns -1 if there is no such integer. Another method is add(value) this will insert value to the queue.

So, if the input is like

• Initialize with [2,3,4] then call the functions as follows −

• showFirstUnique()

• add(5)

• showFirstUnique()

• add(2)

• showFirstUnique()

• add(3)

• showFirstUnique(),

then the output will be 2, 2, 3, -1 respectively.

To solve this, we will follow these steps −

• Define one queue q

• Define one map cnt

• The initializer will take the array

  • for each element i in nums

    • (increase cnt[i] by 1)

  • for each element i in nums

    • if cnt[i] is same as 1, then −

      • insert i into q

• Define a function showFirstUnique()

• while (not q is empty and cnt[first element of q] > 1), do −

  • delete element from q

• return (if q is empty, then -1, the otherwise first element of q)

• Define a function add(), this will take value,

• (increase cnt[value] by 1)

• if cnt[value] is same as 1, then −

  • insert value into q

Example 

Let us see the following implementation to get a better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class FirstUnique {
public:
   queue <int> q;
   map <int, int> cnt;
   FirstUnique(vector<int>& nums) {
      for (int i : nums) {
         cnt[i]++;
      }
      for (int i : nums) {
         if (cnt[i] == 1) {
            q.push(i);
         }
      }
   }
   int showFirstUnique() {
      while (!q.empty() && cnt[q.front()] > 1) q.pop();
         return q.empty() ? -1 : q.front();
   }
   void add(int value) {
      cnt[value]++;
      if (cnt[value] == 1)
         q.push(value);
   }
};
main(){
   vector<int> v = {2,3,5};
   FirstUnique ob(v);
   cout << (ob.showFirstUnique()) << endl;
   ob.add(5);
   cout << (ob.showFirstUnique()) << endl;
   ob.add(2);
   cout << (ob.showFirstUnique()) << endl;
   ob.add(3);
   cout << (ob.showFirstUnique()) << endl;
}

Input

{2,3,5}
ob.showFirstUnique();
ob.add(5);
ob.showFirstUnique();
ob.add(2);
ob.showFirstUnique();
ob.add(3);
ob.showFirstUnique();

Output

2
2
3
-1