middle-of-the-linked-list.py

# Time:  O(n)
# Space: O(1)

# Given a non-empty, singly linked list with head node head,
# return a middle node of linked list.
#
# If there are two middle nodes, return the second middle node.
#
# Example 1:
#
# Input: [1,2,3,4,5]
# Output: Node 3 from this list (Serialization: [3,4,5])
# The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
# Note that we returned a ListNode object ans, such that:
# ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
# Example 2:
#
# Input: [1,2,3,4,5,6]
# Output: Node 4 from this list (Serialization: [4,5,6])
# Since the list has two middle nodes with values 3 and 4, we return the second one.
#
# Note:
# - The number of nodes in the given list will be between 1 and 100.
                                 
# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution(object):
    def middleNode(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow, fast = head, head
        while fast and fast.next:
            slow, fast = slow.next, fast.next.next
        return slow