链表---详解

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本文详细介绍了链表的基础概念，包括头节点的使用、无头链表的实现，以及如何通过快慢指针和哈希表检测循环链表。同时探讨了双向链表的特点和LeetCode中的一道相关问题.

链表

链表不一定由结构体构成，也可能是数与数之间的固定指向

一般来说，有头链表使用时更方便

无头链表代码演示：

#include <iostream>

#include<map>

#include<ctime>

#include<cstdio>

#include<vector>

#include<cstdlib>

#include<unordered_map>

#include<stack>

using namespace std;

#define DIGIT_LEN 3

typedef struct Node {

int data;

Node* next;

}Node;

Node* getNewNode(int val) {

Node* p = (Node*)malloc(sizeof(Node));

p->data = val;

p->next = NULL;

return p;

}

void clear(Node* head) {

if (head == NULL) return;

for (Node* p = head, *q; p; p = q) {

q = p->next;

free(p);

}

return;

}

Node* insert(Node* head, int pos, int val) {

if (pos == 0) {

Node* p = getNewNode(val);

p->next = head;

return p;

}

Node* p = head;

for (int i = 0; i < pos - 1; i++) {

p=p->next;

}

Node* node = getNewNode(val);

node->next = p->next;

p->next = node;

return head;

}

Node* erase(Node* head, int pos) {

Node* p = head;

if (pos == 0) {

head->data = 0;

head = head->next;

p->next = NULL;

return head;

}

for (int i = 0; i < pos - 1; i++)

{

p = p->next;

}

p = p->next->next;

return head;

}

void output_linklist(Node *head,int flag=0) {

int n = 0;

for (Node* p = head; p; p = p->next) {

++n;

}

for (int i = 0; i < n; i++) {

printf("%3d", i);

printf(" ");

}

printf("\n");

for (Node* p = head; p; p = p->next) {

printf("%3d", p->data);

if(p->next)printf("->");

}

if (flag == 0)printf("\n\n\n");

else printf("\n");

return;

}

int find(Node* head, int val) {

Node* p = head;

int n = 0;

while (p) {

if (p->data == val)

{

output_linklist(head,1);

for (int i = 0; i < n; i++)

printf(" ");

printf(" ^ \n");

for (int i = 0; i < n; i++)

printf(" ");

printf(" | \n");

return 1;

}

n += 1;

p = p->next;

}

return 0;

}

int main()

{

srand((unsigned int)time(0));

#define MAX_OP 20

Node* head = NULL;

for (int i = 0; i < MAX_OP; i++) {

int pos = rand() % (i + 1), val = rand() % 100;

printf("insert %d at %d to linklist\n", val, pos);

head=insert(head, pos, val);

output_linklist(head);

}

int val;

while (~scanf_s("%d", &val)) {

if (!find(head, val)) {

printf("not found\n");

}

clear(head);

return 0;

}

有头节点运用于插入--减少了if

Node* insert(Node* head, int pos, int val) {

Node new_head, * p = &new_head, * node = getNewNode(val);//临时创建头节点

new_head.next = head;

for (int i = 0; i < pos; i++) p = p->next;

node->next = p->next;

p->next = node;

return new_head.next;

}

循环链表

特点：

把链表的尾节点当作虚拟头节点
通过快慢指针可以分辨是单向链表还是循环链表

代码：哈希

bool hasCycle(ListNode *head) {

unordered_map<ListNode*,int> umap;

ListNode *p=head;

while(p){

umap[p]++;

if(umap[p]==2) return true;

p=p->next;

}

return false;

}

快慢指针

bool hasCycle(ListNode *head) {

ListNode *fast_ptr=head;

ListNode *low_ptr=head;

while(fast_ptr&&fast_ptr->next){

low_ptr=low_ptr->next;

fast_ptr=fast_ptr->next->next;

if(low_ptr==fast_ptr) return true;

}

return false;

}

双指针等距移动法：确定链表中的倒数第几位的位置

leetcode:19

双向链表