Reverse Linked List II

Description

Reverse a linked list from position m to n. Do it in one-pass.

Note: 1 ≤ m ≤ n ≤ length of list.

Example:

Input: 1->2->3->4->5->NULL, m = 2, n = 4
Output: 1->4->3->2->5->NULL

Solution(javascript)

const reverseBetween = function (head, m, n) {
  let count = 1
  let current = head
  let part1Tail = null
  while (current && count < m) {
    if (count === m - 1) {
      part1Tail = current
    }
    count += 1
    current = current.next
  }
  let prev = null
  let part2Tail = null
  while (current && count <= n) {
    if (!part2Tail) {
      part2Tail = current
    }
    const temp = current.next
    current.next = prev
    prev = current
    current = temp
    count += 1
  }
  if (part1Tail) {
    part1Tail.next = prev
  }
  if (part2Tail) {
    part2Tail.next = current
  }
  return part1Tail ? head : (prev || current)
}