876. Middle of the Linked List

Given the head of a singly linked list, return the middle node. If there are two middle nodes, return the second one.

Move slow one step and fast two steps at a time. When fast runs off the end, slow is in the middle.

1function middleNode(head: ListNode | null): ListNode | null {
2  let slow = head, fast = head;
3  while (fast && fast.next) {
4    slow = slow!.next;
5    fast = fast.next.next;
6  }
7  return slow;
8}

list

slow,fast

→

→ ∅

Both pointers start at the head.

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Space: play/pause · ←/→: step