How to generate the order if you know the next item for each item? - Mailing list pgsql-sql

I have two tables one is a lookup for all the tasks, another table stores the next task for each task, I want to generate a list of tasks with its sequence number, can I do it in one query? 

Table task

id  name 

1   postoffice

2   grocery store

3.  walk dog

4.  swimming 

5   roller skate

7   biking

table next_task

task   next_task

3         7

7         1

1          4

4          2

2          5

and I want to get the list in order 

task                task_order 

3 (walk dog)              1

7 (biking)                   2

1 (postoffice)             3  

4 (swimming)             4

2 (growcery store)      5

5 (roller skate)            6

It might be just very simple and I have tried but somehow I am blocked. 

Thanks in advance

-Rockdale