LeetCode //C - 61. Rotate List

61. Rotate List

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
• $0<=k<=2\ast 10^9$

From: LeetCode
Link: 61. Rotate List

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Solution:

Ideas：

1. Find the Length: First, traverse the list to find its length n.
2. Calculate Effective Rotation: Since rotating a list of length n places is the same as not rotating it at all, we only need to rotate k mod n places.
3. Find New Head: Traverse the list to the (n−k mod n)th node. This will be the new tail after rotation.
4. Perform Rotation: Update the next pointer of the new tail to NULL and set the next pointer of the old tail to the old head.

Code:

struct ListNode* rotateRight(struct ListNode* head, int k) {
    if (head == NULL || k == 0) {
        return head;
    }
    
    // Step 1: Find the length of the list
    int n = 1;
    struct ListNode *tail = head;
    while (tail->next != NULL) {
        n++;
        tail = tail->next;
    }
    
    // Step 2: Calculate the effective number of rotations needed
    k = k % n;
    if (k == 0) {
        return head;
    }
    
    // Step 3: Find the new head and tail
    struct ListNode *new_tail = head;
    for (int i = 0; i < n - k - 1; i++) {
        new_tail = new_tail->next;
    }
    struct ListNode *new_head = new_tail->next;
    
    // Step 4: Perform the rotation
    new_tail->next = NULL;
    tail->next = head;
    
    return new_head;
}