Reorder List

LeetCode 143 • Medium • L0→Ln→L1→Ln-1→…

In-place reorder: 1. Find middle (fast/slow) → 2. Reverse second half → 3. Merge alternately.

TimeO(n)3 passes

SpaceO(1)in-place

Phase: — Step: —

Slow / First

Fast

Second

First

—

Second

—

Speed:

Ready

Press Play. Step 1: Find middle with slow/fast. Step 2: Split & reverse second half. Step 3: Merge first and second alternately.

✎ Whiteboard

Size:3

⌨ Type It

Practice until you don't need to look. Use the guide comments below as scaffolding. The green highlights are the nuances to burn into memory.

# ─── REORDER LIST (LeetCode 143) ───

# Pattern: Find middle → reverse second → merge alternately

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

# 1. Find middle: slow, fast = head, head

# while fast.next and fast.next.next: slow, fast = slow.next, fast.next.next

# 2. Split: second = slow.next; slow.next = None

# 3. Reverse second half (prev, curr loop)

# 4. Merge: first.next=second, second.next=next_first, alternating

# Vars: slow, fast, second, prev, curr

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