"""Leetcode 876. Middle of the Linked List
Easy

URL: https://leetcode.com/problems/middle-of-the-linked-list/

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, val):
        self.val = val
        self.next = None


class Solution(object):
    def middleNode(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        pass


def main():
    pass


if __name__ == '__main__':
    main()

"""Leetcode 876. Middle of the Linked List
Easy

URL: https://leetcode.com/problems/middle-of-the-linked-list/

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, val):
        self.val = val
        self.next = None


class SolutionFastSlow(object):
    def middleNode(self, head):
        """
        :type head: ListNode
        :rtype: ListNode

        Time complexity: O(n).
        Space complexity: O(1).
        """
        fast = slow = head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
        return slow


def main():
    # Input: [1,2,3,4,5]
    # Output: Node 3 from this list (Serialization: [3,4,5])
    head = ListNode(1)
    head.next = ListNode(2)
    head.next.next = ListNode(3)
    head.next.next.next = ListNode(4)
    head.next.next.next.next = ListNode(5)
    print SolutionFastSlow().middleNode(head).val

    # Input: [1,2,3,4,5,6]
    # Output: Node 4 from this list (Serialization: [4,5,6])
    head = ListNode(1)
    head.next = ListNode(2)
    head.next.next = ListNode(3)
    head.next.next.next = ListNode(4)
    head.next.next.next.next = ListNode(5)
    head.next.next.next.next.next = ListNode(6)
    print SolutionFastSlow().middleNode(head).val


if __name__ == '__main__':
    main()

Complete fast slow sol w/ time/space complexity