call chain

[jasen]

There was 25 kids in a class, every kids had some best friends.
To simplify the problem, Let's call each kids with their initial name.
here is a list of kids best friends.

a : [l,h]
b : [k,m,i]
c : [f,l,n,j]
d : [g,m,o]
e : [h,n]
f : [c,m,q]
g : [d,n,r,p]
h : [a,k,q,s,o,e]
i : [b,l,r,t]
j : [c,m,s]
k : [u,r,h,b]
l : [u,w,s,i,c,a]
m : [f,b,d,j,t,x,p,v]
n : [y,w,q,g,c,e]
o : [x,r,h,d]
p : [w,m,g]
q : [x,n,h,f]
r : [u,k,g,i,o,y]
s : [v,l,h,j]
t : [w,m,i]
u : [l,r]
v : [k,m,s]
w : [p,l,n,t]
x : [q,m,o]
y : [r,n]

One day all kids was outside the class room except "a".
then his teacher asked "a" to call one of his best friend to enter the room, "a" called "l". 
then the teacher asked "l" to call one of his best friend outside to enter the room, and so on.
after all the call chain, there was no more kids outside, and "s" was the last kids called.

determine the call chain !

[araver]

Spoiler

Greedy algorithm: identify and use the letter that only has a pair left - an in-node, an out-node.

a called l
l called u
u called r
r called y
y called n
n called e
e called h
h called k
k called b
b called i
i called t
t called w
w called p
p called g
g called d
d called o
o called x
x called q
q called f
f called c
c called j
j called m
m called v
v called s