刷题19. Remove Nth Node From End of List

一、题目说明

这个题目是19. Remove Nth Node From End of List，不言自明。删除链表倒数第n个元素。难度是Medium！

二、我的解答

链表很熟悉了，直接写代码。

性能如下：

Runtime: 8 ms, faster than 35.76% of C++ online submissions for Remove Nth Node From End of List.
Memory Usage: 8.8 MB, less than 5.26% of C++ online submissions for Remove Nth Node From End of List.

#include<iostream>
using namespace std;

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution{
	public:
		ListNode * removeNthFromEnd(ListNode* head,int n){
			if(head==NULL) return NULL;
			if(n<0) return NULL;
			int cur = n;
			ListNode*p = head;
			ListNode* nTh = p;
			while(cur>0 && nTh!=NULL){
				nTh = nTh->next;
				cur--;
			}
			//n超过链表长度
			if(nTh==NULL && cur>0) return head;
			//删除第1个元素
			if(nTh==NULL && cur==0){
				ListNode * t = p->next;
				if(t!=NULL){
					head = p->next;
					delete p;
					return head;
				}else{
					delete p;
					return NULL;
				}
			}

			while(p!=NULL && nTh!=NULL && nTh->next!=NULL){
				p=p->next;
				nTh = nTh->next;
			}
			if(p!=NULL){
				ListNode * tmp = p->next;
				if(p->next !=NULL){
					p->next = tmp->next;
				}

				delete tmp;
			}
			return head;
		}
};
int main(){
	Solution s;
	ListNode dummy(0);
	ListNode *p;
	int i = 5;
	while(i>0){
		ListNode *tmp = new ListNode(i);
		tmp->next = dummy.next;
		dummy.next = tmp;
		i--;
	}
	p = dummy.next;
	while(p!=NULL){
		cout<<p->val<<" ";
		p=p->next;
	}
	cout<<endl;

	ListNode*r = s.removeNthFromEnd(dummy.next,2);
	p = r;
	while(p!=NULL){
		cout<<p->val<<" ";
		p=p->next;
	}
	cout<<endl;

	return 0;
}

三、改进

删除一个变量，性能大幅提高：

Runtime: 4 ms, faster than 88.76% of C++ online submissions for Remove Nth Node From End of List.
Memory Usage: 8.8 MB, less than 5.26% of C++ online submissions for Remove Nth Node From End of List.

改进后代码如下：

#include<iostream>
using namespace std;

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution{
	public:
		ListNode * removeNthFromEnd(ListNode* head,int n){
			if(head==NULL) return NULL;
			if(n<0) return NULL;
			int cur = n;
			ListNode*p = head;
			ListNode* nTh = p;
			while(cur>0 && nTh!=NULL){
				nTh = nTh->next;
				cur--;
			}
			//n超过链表长度
			if(nTh==NULL && cur>0) return head;
			//删除第1个元素
			if(nTh==NULL && cur==0){
				if(p->next!=NULL){
					head = p->next;
					delete p;
					return head;
				}else{
					delete p;
					return NULL;
				}
			}

			while(p!=NULL && nTh!=NULL && nTh->next!=NULL){
				p=p->next;
				nTh = nTh->next;
			}
			if(p!=NULL){
				ListNode * tmp = p->next;
				if(p->next !=NULL){
					p->next = tmp->next;
				}

				delete tmp;
			}
			return head;
		}
};
int main(){
	Solution s;
	ListNode dummy(0);
	ListNode *p;
	int i = 5;
	while(i>0){
		ListNode *tmp = new ListNode(i);
		tmp->next = dummy.next;
		dummy.next = tmp;
		i--;
	}
	p = dummy.next;
	while(p!=NULL){
		cout<<p->val<<" ";
		p=p->next;
	}
	cout<<endl;

	ListNode*r = s.removeNthFromEnd(dummy.next,2);
	p = r;
	while(p!=NULL){
		cout<<p->val<<" ";
		p=p->next;
	}
	cout<<endl;

	return 0;
}

再次改进：

class Solution{
	public:
		ListNode * removeNthFromEnd(ListNode* head,int n){
			if(head==NULL) return NULL;
			if(n<0) return NULL;
			int len = 0;
			ListNode*p = head;
			while(p!=NULL){
				p = p->next;
				len++;
			}
			//n超过链表长度
			if(len<n) return head;
			//删除第1个元素
			if(len==n){
				head = head->next;
				return head;
			}

			int t = len -n -1;
			p=head;
			while(t-->0){
				p=p->next;
			}
			p->next = p->next->next;

			return head;
		}
};