0092.go

// Source: https://leetcode.com/problems/reverse-linked-list-ii
// Title: Reverse Linked List II
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Given the head of a singly linked list and two integers left and right where left <= right, reverse the nodes of the list from position left to position right, and return the reversed list.
//
// Example 1:
//
//   https://assets.leetcode.com/uploads/2021/02/19/rev2ex2.jpg
//
//   Input: head = [1,2,3,4,5], left = 2, right = 4
//   Output: [1,4,3,2,5]
//
// Example 2:
//
//   Input: head = [5], left = 1, right = 1
//   Output: [5]
//
// Constraints:
//
//   The number of nodes in the list is n.
//   1 <= n <= 500
//   -500 <= Node.val <= 500
//   1 <= left <= right <= n
//
// Follow up: Could you do it in one pass?
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

type ListNode struct {
	Val  int
	Next *ListNode
}

// Use iteration
func reverseBetween(head *ListNode, left int, right int) *ListNode {
	preHead := &ListNode{
		Next: head,
	}

	preLeft := preHead
	for i := 1; i < left; i++ {
		preLeft = preLeft.Next
	}
	oldLeft := preLeft.Next

	prev := preLeft
	curr := oldLeft

	for i := left; i <= right; i++ {
		next := curr.Next
		curr.Next = prev
		prev, curr = curr, next
	}
	preLeft.Next = prev
	oldLeft.Next = curr

	return preHead.Next
}