61 Rotate List

Given a list, rotate the list to the right by k places, where k is non-negative.
For example:

Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

Solution

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    int length(ListNode* head) {
        int i = 0;
        ListNode *p = head;
        while ( p != NULL ) {
            p = p->next;
            i += 1;
        }
        return i;
    }
    ListNode* Nth(ListNode* head, int n) {
        ListNode *p = head;
        for ( int i = 1; i <= n-1; i++ ) p = p->next;
        return p;
    }
    ListNode* rotateRight(ListNode* head, int k) {
        int n = length(head);
        if ( n == 0 ) return head;
        k = k%n;
        if ( k == 0 ) return head;
        ListNode *p1 = Nth(head, n-k), *ptail = Nth(head, n);
        ListNode *p2 = p1->next;
        p1->next = NULL;
        ptail->next = head;
        return p2;
    }
};

Notes
Take of the special case:

• n == 0
• k == 0

  p1 == ptail
  p2 == NULL