Please Explain Compute the worst case time complexity of the

Please Explain.

Compute the worst case time complexity of the following algorithm. for i = n to 2n do for j = i - n to n do print (i, j).

Solution

for i=n to 2n do
   for j=i-n to n do
       print (i, j)

Here we have two loops:
   1. outer for loop: runs for i=n to 2n : n times
   2. inner for loop:
           for each value of i, it run :
               0 to n
               1 to n
               2 to n
               .......
               .......
               n-1 to n
               n to n

So, T(n): n + (n-1) + (n-2)+ .......+1

       = n(n+1)/2

   Big-O : O(n^2)