Find missing number in a sequence not sorted

Question

I have to find a missing number in a sequence not sorted. This sequence is stored in a String object. For example, in this sequence: 3 1 6 5 2 the missing number is 4. Between every number there is a \n. I have to do this without using structures as Array, Dictionary, Lists etc because I need to have a O(1) complexity. In input i receive also the max number of the sequence (in the example sequence, i receive the number 6) Any idea?

Answer 1

METHOD 1(Use sum formula) Algorithm:

1. Get the sum of numbers total = n*(n+1)/2 -----------O(1)

2 Subtract all the numbers from sum and you will get the missing number

 void ans(int total){     ///O(1)

            big_total = (n+1)*(n+2)/2;  //  n+1 because 1 number is missing

            return (big_total-total);


     }




input_array()
{
  int n,total=0,i;

  cout<<"enter no of element";
  cin>>n;
  for(i=0;i<n;i++)
   {
     cin>>arr[i];
     total+=arr[i];
   }

   cout<<ans(total);

}