题解 | #调整牛群顺序#

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @param n int整型 
     * @return ListNode类
     */
    ListNode* moveNthToEnd(ListNode* head, int n) {
        // write code here
        if(!head || !head->next){
            return head;
        }
        int N = 0;
        auto *cur = head;
        ListNode *end;
        while(cur !=nullptr){
            N ++;
            if(cur->next == nullptr)
                end = cur;
            cur = cur->next;
        }
        cur = head;
        ListNode * pre = nullptr;
        int target_index = N - n + 1;
        int index = 0;
        while(cur != nullptr){
            index ++;
            if(index == target_index){
                if(pre == nullptr){
                    head = cur -> next;
                    cur->next = nullptr;
                    end->next = cur;
                }else if(cur == end){
                    break;
                }else{
                    pre->next = cur->next;
                    cur->next = nullptr;
                    end->next = cur;
                }
                break;
            }
            pre = cur;
            cur = cur->next;
        }
        return head;
    }
};