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authorVincent Ambo <mail@tazj.in>2021-12-13T22·51+0300
committerVincent Ambo <mail@tazj.in>2021-12-13T23·15+0300
commit019f8fd2113df4c5247c3969c60fd4f0e08f91f7 (patch)
tree76a857f61aa88f62a30e854651e8439db77fd0ea /users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py
parent464bbcb15c09813172c79820bcf526bb10cf4208 (diff)
parent6123e976928ca3d8d93f0b2006b10b5f659eb74d (diff)
subtree(users/wpcarro): docking briefcase at '24f5a642' r/3226
git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c09813172c79820bcf526bb10cf4208
git-subtree-split: 24f5a642af3aa1627bbff977f0a101907a02c69f
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789
Diffstat (limited to 'users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py')
-rw-r--r--users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py70
1 files changed, 70 insertions, 0 deletions
diff --git a/users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py b/users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py
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index 000000000000..523ecd959daf
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+++ b/users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py
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+def contains_cycle(node):
+    """
+    Return True if the linked-list, `node`, contains a cycle.
+    """
+    if not node:
+        return False
+    a = node
+    b = node.next
+    while a != b:
+        a = a.next
+        if b and b.next and b.next.next:
+            b = b.next.next
+        else:
+            return False
+    return True
+
+################################################################################
+# Bonus
+################################################################################
+
+def first_node_in_cycle(node):
+    """
+    Given that the linked-list, `node`, contains a cycle, return the first
+    element of that cycle.
+    """
+    # enter the cycle
+    a = node
+    b = node.next
+    while a != b:
+        a = a.next
+        b = b.next.next
+
+    # get the length of the cycle
+    beg = a
+    a = a.next
+    n = 1
+    while a != beg:
+        a = a.next
+        n += 1
+
+    # run b n-steps ahead of a
+    a = node
+    b = node
+    for _ in range(n):
+        b = b.next
+
+    # where they intersect is the answer
+    while a != b:
+        a = a.next
+        b = b.next
+    return a
+
+################################################################################
+# Tests
+################################################################################
+
+class Node(object):
+    def __init__(self, value, next=None):
+        self.value = value
+        self.next = next
+    def __repr__(self):
+        return "Node({}) -> ...".format(self.value)
+
+d = Node('d')
+c = Node('c', d)
+b = Node('b', c)
+a = Node('a', b)
+d.next = b
+
+print(first_node_in_cycle(a))