My code is like this and the input is here.

5 6

1 2

1 5

2 4

3 4

3 5

4 5

when i put 2 4 , the value root_a is None. but i don't understand why root_a return None . i tried to debug on pycharm but i don't know why... Does anyone can help?

node, edge = list(map(int, input().split()))
root_list = [x for x in range(node)]


def find_root(x):
    print(x, 'x')
    if x == root_list[x]:
        return x
    root_x = find_root(root_list[x])
    root_list[x] = root_x


for _ in range(edge):
    a, b = list(map(int, input().split()))
    a = a - 1
    b = b - 1
    root_a = find_root(a)
    root_b = find_root(b)
    print(root_a, root_b, 'root')
    if root_a != root_b:
        root_list[b] = root_a
    print(root_list)

CodePudding user response：

I apologize if I understand your code wrongly. I'm not quite sure what you are trying to achieve ;(

But I can tell you why it's None when you input 2 4

that is because in the function find_root you are returning nothing if x != root_list[x]

see below for find_root function with comments

def find_root(x):
    print(x, 'x')
    if x == root_list[x]:
        return x
    # here if x != root_list[x], the code will continue
    # but there is no return, hence it's treated as if it returns None
    root_x = find_root(root_list[x])
    root_list[x] = root_x
    # you might need a return here?
    return root_x

CodePudding user response：

You don't return the found value in find_root. If you return no value 'None' is returned.

In the third iteration of your loop _ == 2 you call find_root(1) which then calls find_root(0) but that value isn't returned after being assigned to root_x. Instead None is returned implicitly and root_a becomes None. As None != 3 you assign None into your root_list in line 21.

I'm unsure what you program wants to do but try this:

def find_root(x):
    print(x, 'x')
    if x == root_list[x]:
        return x
    root_x = find_root(root_list[x])
    root_list[x] = root_x
    return root_x # add this line