Link $1$st node's ptr2 pointer to $2$nd node. then $2$nd node to $4$th node. $4$th node to $8$th node, $8$th node to $16$th so on.
For example I need $100$th node then do like this- go to $1\rightarrow 2 \rightarrow 4 \rightarrow 8 \rightarrow 16 \rightarrow 32 \rightarrow 64 \rightarrow 128$. Then it takes $\log i$ time.
Then to go to $100$ start at $64$ and linear traversal has to be done for $36$ times.
This linear traversal should take $i/2$ node traversals in the worst case.
Overall it would take traversing no more than $\log i + i/2 $ nodes.