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Grokking the Coding Interview Patterns

Reorder List

Explore how to reorder a singly linked list by folding it onto itself using in-place link manipulation. This lesson helps you understand and implement efficient linked list reordering to solve common coding interview problems involving node rearrangement.

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Statement

Given the head of a singly linked list, reorder the list as if it were folded on itself. For example, if the list is represented as follows:

L0L_{0} → L1L_{1} → L2L_{2} → … → Ln−2L_{n-2} → Ln−1L_{n-1} → LnL_{n} ​

This is how you’ll reorder it:

L0L_{0} → LnL_{n} → L1L_{1} → Ln−1L_{n - 1} → L2L_{2} → Ln−2L_{n - 2} → …

You don’t need to modify the values in the list’s nodes; only the links between nodes need to be changed.

Constraints:

  • The range of number of nodes in the list is [1,500][1, 500].
  • −5000≤-5000 \leq Node.value ≤5000\leq 5000

Examples

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Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Reorder List

1.

What is the output if the following linked list is given as input?

4 → 2 → 7 → 8 → 9 → 0 → 2

A.

2 → 4 → 2 → 0 → 7 → 9 → 8

B.

4 → 2 → 7 → 2 → 8 → 0 → 9

C.

9 → 0 → 2 → 8 → 4 → 2 → 7

D.

4 → 2 → 2 → 0 → 7 → 9 → 8

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Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
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Try it yourself

Implement your solution in the following coding playground.

Java
usercode > Solution.java
// Definition for a Linked List node
// class ListNode {
//     int val;
//     ListNode next;
//     // Constructor
//     public ListNode(int val) {
//         this.val = val;
//         this.next = null;
//     }
// }
import ds_v1.LinkedList.ListNode;
import java.util.*;
public class Solution
{
    public static ListNode reorderList(ListNode head) {
    // Replace this placeholder return statement with your code
    return head;
  }
}
Reorder List