john smith - 8 months ago 57

Python Question

I am solving a problem in which I need to calculate the diameter of the tree.I know how to calculate that using 2 bfs first to find the farthest node then do second bfs using the node we found from the first one.

But I am having difficulty to implement a very simple step - making a adjacency list (dict in case of python) from the input I have written a code to this but its not tidy and not the best can someone tell how to do this efficently

Input

The first line of the input file contains one integer N --- number of

nodes in the tree (0 < N <= 10000). Next N-1 lines contain N-1 edges

of that tree --- Each line contains a pair (u, v) means there is an

edge between node u and node v (1 <= u, v <= N).

Example:

8

1 2

1 3

2 4

2 5

3 7

4 6

7 8

My code is :

`def makedic(m):`

d = {}

for i in range(m):

o, p = map(int, raw_input().split())

if o not in d and p not in d:

d[p] = []

d[o] = [p]

elif o not in d and p in d:

d[o] = []

d[p].append(o)

elif p not in d and o in d:

d[p] = []

d[o].append(p)

elif o in d:

d[o].append(p)

# d[p].append(o)

elif p in d:

d[p].append(o)

return d

Here is how I implemented bfs:

`def bfs(g,s):`

parent={s:None}

level={s:0}

frontier=[s]

ctr=1

while frontier:

next=[]

for i in frontier:

for j in g[i]:

if j not in parent:

parent[j]=i

level[j]=ctr

next.append(j)

frontier=next

ctr+=1

return level,parent

Answer

There are unnecessary checks in your code. For each *A - B* edge you just have to put *B* in *A*'s adjacency list and *A* in *B*'s adjacency list:

```
d = {}
for i in range(m):
u,v = map(int, raw_input().split())
if u in d:
d[u].append(v)
else:
d[u] = [v]
if v in d:
d[v].append(u)
else:
d[v] = [u]
```

According to the question every node has an index between *1* and *N* so you can use this fact and pre-populate the `dict`

with empty lists. This way you don't have to check whether a key is in the `dict`

or not. Also make the code a little bit shorter:

```
N = input()
d = { i:[] for i in range(1, N+1) }
for i in range(N):
u,v = map(int, raw_input().split())
d[u].append(v)
d[v].append(u)
```

Source (Stackoverflow)