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mediumDSA

How do you find the middle of a linked list?

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01

Understand the problem

Explain the slow/fast technique.

linked-listtwo-pointers
02

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Sketch your approach before reading the solution — that's what interviews test.

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03

Study the solution

Use slow and fast pointers: advance slow by one and fast by two. When fast reaches the end, slow is at the middle — one pass, O(n) time, O(1) space.

Solution ready — 2 min read

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04

Read the code

Slow / fast pointers
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class ListNode:
    def __init__(self, val, nxt=None):
        self.val, self.next = val, nxt

def middle_node(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    return slow


# --- demo ---
def build(vals):
    head = None
    for v in reversed(vals): head = ListNode(v, head)
    return head

print(middle_node(build([1, 2, 3, 4, 5])).val)    # 3
print(middle_node(build([1, 2, 3, 4, 5, 6])).val) # 4 (second middle)
05

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