Reorder List Approach

Step 1: Find middle using fast-slow pointers.

Step 2: Reverse the second half.

Step 3: Merge alternately.

# Step 1: Find middle
slow, fast = head, head
while fast.next and fast.next.next:
    slow = slow.next
    fast = fast.next.next
second = slow.next
slow.next = null

# Step 2: Reverse second half
second = reverse(second)

# Step 3: Merge alternately
first = head
while second:
    tmp1, tmp2 = first.next, second.next
    first.next = second
    second.next = tmp1
    first, second = tmp1, tmp2

Time: $O(n)$. Space: $O(1)$.