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143. Reorder List 👍

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  void reorderList(ListNode* head) {
    if (!head || !head->next)
      return;

    ListNode* mid = findMid(head);
    ListNode* reversed = reverse(mid);
    merge(head, reversed);
  }

 private:
  ListNode* findMid(ListNode* head) {
    ListNode* prev = nullptr;
    ListNode* slow = head;
    ListNode* fast = head;

    while (fast != nullptr && fast->next != nullptr) {
      prev = slow;
      slow = slow->next;
      fast = fast->next->next;
    }
    prev->next = nullptr;

    return slow;
  }

  ListNode* reverse(ListNode* head) {
    ListNode* prev = nullptr;
    ListNode* curr = head;

    while (curr != nullptr) {
      ListNode* next = curr->next;
      curr->next = prev;
      prev = curr;
      curr = next;
    }

    return prev;
  }

  void merge(ListNode* l1, ListNode* l2) {
    while (l2) {
      ListNode* next = l1->next;
      l1->next = l2;
      l1 = l2;
      l2 = next;
    }
  }
};

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class Solution {
  public void reorderList(ListNode head) {
    if (head == null || head.next == null)
      return;

    ListNode mid = findMid(head);
    ListNode reversed = reverse(mid);
    merge(head, reversed);
  }

  private ListNode findMid(ListNode head) {
    ListNode prev = null;
    ListNode slow = head;
    ListNode fast = head;

    while (fast != null && fast.next != null) {
      prev = slow;
      slow = slow.next;
      fast = fast.next.next;
    }
    prev.next = null;

    return slow;
  }

  private ListNode reverse(ListNode head) {
    ListNode prev = null;
    ListNode curr = head;

    while (curr != null) {
      ListNode next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }

    return prev;
  }

  private void merge(ListNode l1, ListNode l2) {
    while (l2 != null) {
      ListNode next = l1.next;
      l1.next = l2;
      l1 = l2;
      l2 = next;
    }
  }
}

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class Solution:
  def reorderList(self, head: ListNode) -> None:
    def findMid(head: ListNode):
      prev = None
      slow = head
      fast = head

      while fast and fast.next:
        prev = slow
        slow = slow.next
        fast = fast.next.next
      prev.next = None

      return slow

    def reverse(head: ListNode) -> ListNode:
      prev = None
      curr = head

      while curr:
        next = curr.next
        curr.next = prev
        prev = curr
        curr = next

      return prev

    def merge(l1: ListNode, l2: ListNode) -> None:
      while l2:
        next = l1.next
        l1.next = l2
        l1 = l2
        l2 = next

    if not head or not head.next:
      return

    mid = findMid(head)
    reversed = reverse(mid)
    merge(head, reversed)