Hi
Refer following example.

 int[] array = { 1, 3, 6, 8, 14, 18 };
        stringbuilder sbr = new stringbuilder();
 for (int i = 0; i < array.length - 1; i++)
 {
 for (int j = array[i]; j < array[i + 1]; j++)
 {
 var temp = j + 1;
 if (temp < array[i + 1])
 {
                    sbr.append(temp);
 }
 }
 }
        console.writeline(sbr.tostring());
        console.read();

int[] A = { 1, 5, 8, 10, 11, 16 };
for (int i = 0; i < A.Length-1; i++)
{
int num = A[i + 1] - A[i];
for (int j = 1; j < num; j++)
{
Console.Write(A[i] + j+” “);
}
}
Console.ReadLine();

int[] array = { 1, 3, 6, 8, 14, 18 };
StringBuilder sbr = new StringBuilder();

 for (int i = 0; i <array.Length-1; i++)
 {
 if (array[i] < array[i + 1])
 {
 int startval = array[i];
 int endval = array[i + 1];
 for (int j = startval+1; j < endval; j++)
 {
                    sbr.Append( j+ " ");
 }
 } 
 } 
 Console.WriteLine(sbr.ToString()); 
 Console.ReadKey();