linked list algorithms in python

python/07_linkedlist/linked_list_algo.py

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+"""
+    1) Reverse singly-linked list
+    2) Detect cycle in a list
+    3) Merge two sorted lists
+    4) Remove nth node from the end
+    5) Find middle node
+
+    Author: Wenru
+"""
+
+from typing import Optional
+
+
+class Node:
+    
+    def __init__(self, data: int, next=None):
+        self.data = data
+        self._next = next
+
+# Reverse singly-linked list
+# 单链表反转
+# Note that the input is assumed to be a Node, not a linked list.
+def reverse(head: Node) -> Optional[Node]:
+    reversed_head = None
+    current = head
+    while current:
+        reversed_head, reversed_head._next, current = current, reversed_head, current._next
+    return reversed_head
+
+# Detect cycle in a list
+# 检测环
+def has_cycle(head: Node) -> bool:
+    slow, fast = head, head
+    while fast and fast._next:
+        slow = slow._next
+        fast = fast._next._next
+        if slow == fast:
+            return True
+    return False
+
+# Merge two sorted linked list
+# 有序链表合并
+def merge_sorted_list(l1: Node, l2: Node) -> Optional[Node]:
+    if l1 and l2:
+        p1, p2 = l1, l2
+        fake_head = Node(None)
+        current = fake_head
+        while p1 and p2:
+            if p1.data <= p2.data:
+                current._next = p1
+                p1 = p1._next
+            else:
+                current._next = p2
+                p2 = p2._next
+            current = current._next
+        current._next = p1 if p1 else p2
+        return fake_head._next
+    return p1 or p2
+
+# Remove nth node from the end
+# 删除倒数第n个节点。假设n大于0
+def remove_nth_from_end(head: Node, n: int) -> Optional[Node]:
+    fast = head
+    count = 0
+    while fast and count < n:
+        fast = fast._next
+        count += 1
+    if not fast and count < n:  # not that many nodes
+        return head
+    if not fast and count == n:
+        return head._next
+    
+    slow = head
+    while fast._next:
+        fast, slow = fast._next, slow._next
+    slow._next = slow._next._next
+    return head
+
+def find_middle_node(head: Node) -> Optional[Node]:
+    slow, fast = head, head
+    fast = fast._next if fast else None
+    while fast and fast._next:
+        slow, fast = slow._next, fast._next._next
+    return slow
+
+def print_all(head: Node):
+    nums = []
+    current = head
+    while current:
+        nums.append(current.data)
+        current = current._next
+    print("->".join(str(num) for num in nums))