Reorder List

Step0 / 29

PhaseFind Middle

Nodes5

Chain[1→2→3→4→5]

Reorder List (Python)

1class Solution:
●    def reorderList(self, head):
3        if not head or not head.next:
......
8        while fast.next and fast.next.next:
9            slow = slow.next
10            fast = fast.next.next
11 
......
13        prev, curr = None, slow.next
14        slow.next = None
15        while curr:
16            tmp = curr.next
17            curr.next = prev
18            prev = curr
19            curr = tmp
......
21        # Phase 3: Merge two halves
22        p1, p2 = head, prev
23        while p2:
24            p1_next, p2_next = p1.next, p2.next
25            p1.next = p2
26            p2.next = p1_next
27            p1 = p1_next
28            p2 = p2_next

Current Line (2): Method Signature

Reorder List Visualizer

Start

Step

0 / 29

Phase

Find Middle

Nodes

Reorder Progress

Current Phase

Find Middle

Reordered Chain

12345

Find Middle

Reverse Half

Merge/Interleave

Done

Press Next to begin.

Pointer State

slow1

fast1

Why This Step Matters

Both slow and fast start at head. fast moves twice as fast.

Current Action

Initialize: slow = 1, fast = 1

Step Explanation

Start

Initialize Pointers

Set slow and fast pointers both to head. We'll use slow/fast to find the midpoint of the list.

> Phase: Find Middle
> slow and fast both start at head.
> slow moves 1 step at a time; fast moves 2 — when fast hits the end, slow is at the middle.

Unvisited

Current (slow/p1)

Fast / p2 / tmp

Reversed

Done / Merged