SQL to update with Unique Values

Question

How will you update records with unique values in table by just using a single update statement ?

e.g. 

Col1
----
 1
 1
 2
 2
 3 

o/p:

Col1
----
 1
 2
 3
 4
 5

Answer 1

Try this

Update table_name set col1 = rownum;

Answer 2

with cte as(
   select col1, row_number() over(order by col1) rno 
   from #table_name
)
update cte set col1 = rno