Two method on Leetcode 25. Reverse Nodes in k-Group

25. Reverse Nodes in k-Group


Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

You may not alter the values in the nodes, only nodes itself may be changed.

Only constant memory is allowed.

For example,
Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5


Solution 1

The first method is using recursion, that intially deal with the last part of link and it is easy to understand.

assume a link with 5 nodes and K=3.

Solution 2

The second solution contrary to the former, deal with the links from head to tail.
In every group we need to locate tail and previous node of head.

assume a link with 5 nodes and K=3.

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