leetCode - Reverse Nodes in k-Group

문제 

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 list's nodes, only nodes themselves may be changed.

 

Example 1:

Input: head = [1,2,3,4,5], k = 2 Output: [2,1,4,3,5]

Example 2:

Input: head = [1,2,3,4,5], k = 3 Output: [3,2,1,4,5]

Example 3:

Input: head = [1,2,3,4,5], k = 1 Output: [1,2,3,4,5]

Example 4:

Input: head = [1], k = 1 Output: [1]

 

Constraints:

• The number of nodes in the list is in the range sz.
• 1 <= sz <= 5000
• 0 <= Node.val <= 1000
• 1 <= k <= sz

 

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풀이

function reverseKGroup(head, k) {
    if (!head) return null;
    let tail = head;
    for (let i = 1; i < k; i++) {
        tail = tail.next;
        if (!tail) return head;
    }

    let next = tail.next;
    tail.next = null;
    reverse(head);
    head.next = reverseKGroup(next, k);
    return tail;
  
};

function reverse(curr) {
    let prev = null;
    while(curr){
        let next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}