[Solved] 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.

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Table of Contents

Question

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:

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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?

Python Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseBetween(self, head: ListNode, left: int, right: int) -> ListNode:
        dummy = ListNode(0)
        dummy.next = head
        
        leftportion = dummy
        
        for _ in range(left-1):
            leftportion = leftportion.next
        
        
        curr = leftportion.next
        x = curr
        prev = None
        
        for _ in range(right-left+1):
            nex = curr.next
            curr.next = prev
            prev,curr = curr,nex
        
        leftportion.next = prev
        x.next = nex
        return dummy.next

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