initially, 1 ---> 2 ---> 3
pre = next = NULL, ptr points to the element 1.
ptr != Null ===> enter into loop
1) making next as element 2
2) making element 1 --> link = prev = NULL
3) making prev as element 1
4) making ptr as element 2
ptr != Null ===> enter into loop
1) making next as element 3
2) making element 2 --> link = prev = element 1
3) making prev as element 2
4) making ptr as element 3
ptr != Null ===> enter into loop
1) making next as NULL due to 3 --> next = NULL
2) making element 3 --> link = prev = element 2
3) making prev as element 3
4) making ptr as NULL
ptr != NULL ===> out of loop.
start = prev ===> making 3 as start of the linked list.
3 ---> 2 ---> 1