19.2.9 [LeetCode 61] Rotate List

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

Example 1:

Input: 1->2->3->4->5->NULL, k = 2
Output: 4->5->1->2->3->NULL
Explanation:
rotate 1 steps to the right: 5->1->2->3->4->NULL
rotate 2 steps to the right: 4->5->1->2->3->NULL

Example 2:

Input: 0->1->2->NULL, k = 4
Output: 2->0->1->NULL
Explanation:
rotate 1 steps to the right: 2->0->1->NULL
rotate 2 steps to the right: 1->2->0->NULL
rotate 3 steps to the right: 0->1->2->NULL
rotate 4 steps to the right: 2->0->1->NULL

题意

把链表循环右移k个

题解

class Solution {
public:
    ListNode* rotateRight(ListNode* head, int k) {
        if (head == NULL)return NULL;
        ListNode*pre, *p = head;
        int cnt = 0;
        while (p != NULL) {
            p = p->next;
            cnt++;
        }
        k %= cnt;
        if (k == 0)return head;
        p = head;
        while (k--)
            p = p->next;
        pre = head;
        while (p->next != NULL) {
            p = p->next;
            pre = pre->next;
        }
        ListNode*ans = pre->next;
        pre->next = NULL;
        p->next = head;
        return ans;
    }
};

考虑空链和不移动的特殊情况