Grokking the Coding Interview: Patterns for Coding Questions

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Solution: Reverse a LinkedList

Problem Statement

Given the head of a Singly LinkedList, reverse the LinkedList. Write a function to return the new head of the reversed LinkedList.

Constraints:

  • The number of nodes in the list is the range [0, 5000].
  • -5000 <= Node.val <= 5000

Solution

To reverse a LinkedList, we need to reverse one node at a time. We will start with a variable current which will initially point to the head of the LinkedList and a variable previous which will point to the previous node that we have processed; initially previous will point to null.

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Thomas MinhTu Hoang

Thomas MinhTu Hoang

· a year ago

// public class ListNode {
//     public int Val = 0;
//     public ListNode Next;

//     public ListNode(int value) {
//         this.Val = value;
//     }
// }

public class Solution {

    public ListNode reverse(ListNode head) {
      if (head?.Next == null) return head;

      ListNode fwd = head;
      ListNode bwd = null;

      while (fwd != null) {
        bwd = new ListNode(fwd.Val) { Next = bwd };
        fwd = fwd.Next;
      }

      return bwd;
    }
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