0061 - Rotate List (Medium)

Problem Link

https://leetcode.com/problems/rotate-list/

Problem Statement

Given the head of a linked list, rotate the list to the right by k places.

Example 1:

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

Example 2:

Input: head = [0,1,2], k = 4
Output: [2,0,1]

Constraints:

• The number of nodes in the list is in the range [0, 500].
• -100 <= Node.val <= 100
• 0 <= k <= 2 * 10^9

Approach 1: Connect and Cut

First we iterate each node till the end and connect the tail to the head. At the same time we calculate how many nodes there, says $n$. After that, we find a point to cut the list. The point to cut is $n - k \mod n$. As $k$ can be greater or equal to $n$, so the new head will be located at $n - k \mod n$. Then we link the new tail->next to null and return the new head.

C++

Written by @wingkwong

class Solution {
public:
    ListNode* rotateRight(ListNode* head, int k) {
        if (!head) return NULL;
        ListNode *p = head;
        int n = 1;
        while (p->next) {
            p = p->next;
            n++;
        }
        // connect tail to head - like a ring
        p->next = head;
        // find the location to cut
        k = len - k % len;
        // move to that location
        while (k--) p = p->next;
        // p->next is the new head
        head = p->next;
        // make it as the new tail
        p->next = NULL;
        // return the new head
        return head;
    }
};

Python

Written by @radojicic23

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def rotateRight(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
        curr = head
        n = 1
        if not head: return None
        while curr.next:
            curr = curr.next
            n += 1
        curr.next = head
        k = n - k % n
        while k:
            curr = curr.next
            k -= 1
        head = curr.next
        curr.next = None
        return head

JavaScript

Written by @radojicic23

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} k
 * @return {ListNode}
 */
var rotateRight = function(head, k) {
    let curr = head;
    let n = 1;
    if (!head) return null;
    while (curr.next) {
        curr = curr.next;
        n++;
    }
    curr.next = head;
    k = n - k % n;
    while (k) {
        curr = curr.next;
        k--;
    }
    head = curr.next;
    curr.next = null;
    return head;
}