LeetCode 143: Reorder List Solution

Master LeetCode problem 143 (Reorder List), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

143. Reorder List

Medium

Linked List Two Pointers Stack Recursion

Problem Explanation

Explanation

To reorder the linked list as required, we can follow these steps:

Find the middle of the linked list using the slow and fast pointer technique.
Reverse the second half of the list.
Merge the first half with the reversed second half.

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

Solution Code

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

        ListNode middle = findMiddle(head);
        ListNode secondHalf = reverseList(middle.next);
        middle.next = null;

        mergeLists(head, secondHalf);
    }

    private ListNode findMiddle(ListNode head) {
        ListNode slow = head, fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    private ListNode reverseList(ListNode head) {
        ListNode prev = null, curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    private void mergeLists(ListNode l1, ListNode l2) {
        while (l1 != null && l2 != null) {
            ListNode l1Next = l1.next;
            ListNode l2Next = l2.next;

            l1.next = l2;
            l2.next = l1Next;

            l1 = l1Next;
            l2 = l2Next;
        }
    }
}

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