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<<-DOC
You are given a single linked list, and you are asked to determine if there is a circle inside it
A circle means, a -> b -> c -> b, then it is a circle
Part 2.
Could you get the length of the circle for the input single linked list if there is one,
or output 0 if there is no circle? (O(1) space and O(n) time complexity)
A -> B -> C -> D -> E -> F -> D
|T|H|
|A|B|
|B|D|
|C|F|
|D|E|
|E|D|
Part 3.
could you find the entry point of the circle?
which means the point A for (a->b->c->a)
DOC
describe "cycle" do
def circle?(head)
return false if head.nil?
tortoise, hare = head, head.next_item
until hare.nil?
return true if tortoise == hare
tortoise, hare = tortoise.next_item, hare.next_item&.next_item
end
false
end
def cycle_length(head)
return 0 if head.nil?
tortoise, hare = head, head.next_item
detected = false
count = 0
until hare.nil?
if tortoise == hare
return count if detected
detected = true
end
count += 1 if detected
tortoise, hare = tortoise.next_item, hare.next_item&.next_item
end
0
end
def cycle_length(head)
return 0 if head.nil?
tortoise, hare = head, head
d1, d2 = 0, 0
while hare && tortoise
break if tortoise == hare && hare != head
tortoise, hare = tortoise.next_item, hare.next_item&.next_item
d1 += 1
d2 += 2
end
return 0 if hare.nil?
d2 - d1
end
def entry_point(head)
return nil if head.nil?
tortoise, hare = head, head.next_item
until hare.nil?
break if tortoise == hare
tortoise, hare = tortoise.next_item, hare.next_item&.next_item
end
tortoise = head
until hare == tortoise
tortoise = tortoise.next_item
hare = hare.next_item
end
return tortoise
end
def entry_point(head)
return nil if head.nil?
tortoise, hare = head, head.next_item
until hare.nil?
break if tortoise == hare
tortoise, hare = tortoise.next_item, hare.next_item&.next_item
end
return nil if tortoise.nil? || hare.nil?
visited = {}
tortoise = head
until hare == tortoise
tortoise = tortoise.next_item
hare = hare.next_item
return hare if visited[hare]
visited[hare] = true
end
return tortoise
end
class Node
attr_reader :next_item, :value
def initialize(value)
@value = value
end
def add(value)
@next_item = value.is_a?(Node) ? value : Node.new(value)
end
def inspect
"#{object_id} #{value}"
end
end
it do
a = Node.new("A")
b = Node.new("B")
c = Node.new("C")
a.add(b)
b.add(c)
c.add(b)
expect(circle?(a)).to be_truthy
expect(cycle_length(a)).to eql(2)
end
<<-DOC
A -> B -> C -> D -> E -> F -> D
|T|H|
|A|B|
|B|D|
|C|F|
|D|E|
|E|D|
DOC
it do
a = Node.new("A")
b = Node.new("B")
c = Node.new("C")
d = Node.new("D")
e = Node.new("E")
f = Node.new("F")
a.add(b)
b.add(c)
c.add(d)
d.add(e)
e.add(f)
f.add(d)
expect(circle?(a)).to be_truthy
expect(cycle_length(a)).to eql(3)
end
it do
a = Node.new("A")
b = Node.new("B")
a.add(b)
expect(circle?(a)).to be_falsey
expect(cycle_length(a)).to eql(0)
end
it 'detects the entry point' do
# (a->b->c->a)
# a|b
# b|a
# c|c
a = Node.new("A")
b = Node.new("B")
c = Node.new("C")
a.add(b)
b.add(c)
c.add(a)
expect(entry_point(a)).to eql(a)
end
it 'detects entry point' do
# A -> B -> C -> D -> C is C the entry point
a = Node.new("A")
b = Node.new("B")
c = Node.new("C")
d = Node.new("D")
a.add(b)
b.add(c)
c.add(d)
d.add(c)
expect(entry_point(a)).to eql(c)
end
it 'finds no entry point' do
#(a->b->c)
a = Node.new("A")
b = Node.new("B")
c = Node.new("C")
a.add(b)
b.add(c)
expect(entry_point(a)).to be_nil
end
end
|
