we can start comparing the elements pair wise and build a decision tree in which case it takes n-1 comparisons to obtain the highest element.
In order to get the second highest element we need to check only those elements which were compared with the highest element while building the decision tree, now as the height of the tree is logn as logn comparisons are required for the highest element to reach the top, so to obtain second highest element one need logn-1 comparisons so all together n-1+logn-1=n+logn-2 comparisons are required.