FastPrep
Problem · Linked List

Reverse a Singly Linked List

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Problem statement

Given the head head of a singly linked list, reverse the direction of every next pointer and return the new head.

The returned list must contain exactly the original nodes and values in reverse order.

Function

reverseList(head: ListNode) → ListNode

Examples

Example 1

head = [1,2,3,4,5]return = [5,4,3,2,1]

Every link is reversed, making 5 the new head and 1 the new tail.

Example 2

head = [1,2]return = [2,1]

The second node becomes the head and points to the first node.

Example 3

head = [7]return = [7]

A one-node list is already reversed.

Constraints

  • 1 <= number of nodes <= 10^5.
  • -10^9 <= node.val <= 10^9.
  • The input list is finite and contains no cycle.

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/**
 * Definition for singly-linked list.
 * class ListNode {
 *   int val;
 *   ListNode next;
 * }
 */
public ListNode reverseList(ListNode head) {
  // write your code here
}
head[1,2,3,4,5]
expected[5,4,3,2,1]
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