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利用C语言实现链表，并定义一些常用的操作

链表定义

使用typedef简化代码

#include <stdio.h>
#include <stdlib.h>typedef struct LinkedList {int data;struct LinkedList *next;
} LinkedList;

新建一个链表结点

LinkedList *createLinkedList(int i) {LinkedList *list = (LinkedList *)malloc(sizeof(LinkedList));list->data = i;list->next = NULL;return list;
}

打印链表

void printAll(LinkedList *head) {LinkedList *current = head;while (current != NULL) {printf(" --->%d", current->data);current = current->next;}printf("\n");
}

插入结点

头插法（常用）

采用头插法产生的链表和输入的数字是逆序的。
例如：一次插入1，2，3，4，5。链表为：5–>4 -->3 -->2 -->1

LinkedList **p 是指向指针的指针，用于直接修改调用者作用域中的头指针。如果采用LinkedList *p那么只能修改指针中的数值，而不能直接修改头指针，那么头插法也就没法实现了。

void insertAtHead(LinkedList **p, int data) {LinkedList *newNode = malloc(sizeof(LinkedList));newNode->data = data;newNode->next = *p;*p = newNode;
}

运行

int main() {LinkedList *head2 = NULL;insertAtHead(&head2, 1);insertAtHead(&head2, 2);insertAtHead(&head2, 3);insertAtHead(&head2, 4);printAll(head2);return 0;
}

尾插法（使用较少）

遍历到最后一个结点，然后把目标结点挂到链表上

void insertAtTail(LinkedList **p, int data) {LinkedList *newNode = malloc(sizeof(LinkedList));newNode->data = data;newNode->next = NULL;if (*p == NULL) {*p = newNode;}else {LinkedList *current = *p;// 遍历到最后一个节点while (current->next != NULL) {current = current->next;}current->next = newNode;}
}

运行

与头插法顺序恰恰相反

int main() {LinkedList *head2 = NULL;insertAtTail(&head2, 1);insertAtTail(&head2, 2);insertAtTail(&head2, 3);insertAtTail(&head2, 4);printAll(head2);return 0;
}

返回链表长度

int lengthOfNode(LinkedList *head) {int length = 0;LinkedList *current = head;while (current != NULL) {length++;current = current->next;}return length;
}

链表转置

采用三指针法，每次循环保存prev和next，实现局部转置，直到全部结点都实现转置。此时current为NULL，prev刚好指向最后一个元素，也就是转置后的第一个元素，使其为头节点即可。

void reverseList(LinkedList **head) {LinkedList *current = *head;LinkedList *prev = NULL;LinkedList *next = NULL;while (current != NULL) {next = current->next; // 保存nextcurrent->next = prev; // 实现局部转置prev = current;current = next;}*head = prev;
}

运行

int main() {LinkedList *head2 = NULL;insertAtHead(&head2, 1);insertAtHead(&head2, 3);insertAtHead(&head2, 5);insertAtHead(&head2, 6);insertAtHead(&head2, 99);printAll(head2);reverseList(&head2);printAll(head2);return 0;
}

合并两个有序的链表

创建一个临时链表，当两个链表都不为空时，比较链表结点大小，总是将值小的链表结点挂到临时链表上，当有一个链表为空时，直接将剩下的那个链表结点全部挂到临时链表即可。

LinkedList *mergeList(LinkedList *a, LinkedList *b) {LinkedList *pHead = malloc(sizeof(LinkedList));pHead->data = -1;LinkedList *current = pHead;while (a!=NULL && b!=NULL) {LinkedList *tmp = malloc(sizeof(LinkedList));if (a->data < b->data) {tmp->data = a->data;current->next = tmp;current = current->next;a = a->next;}else {tmp->data = b->data;current->next = tmp;current = current->next;b = b->next;}}if (a!=NULL) {current->next = a;}if (b!=NULL) {current->next = b;}return pHead->next;
}

运行

int main() {LinkedList *head1 = NULL;insertAtTail(&head1, 1);insertAtTail(&head1, 3);insertAtTail(&head1, 8);printAll(head1);LinkedList *head2 = NULL;insertAtTail(&head2, 2);insertAtTail(&head2, 7);insertAtTail(&head2, 99);printAll(head2);printAll(mergeList(head1, head2));return 0;
}

删除最小结点

先创建一个值为-1的哑结点，为了简化对链表为NULL时的处理。
通过循环找到最小结点，并且记录最小结点的前结点。前结点下一个结点指向最小结点的下一个结点即可（跨过最小结点）

void deleteMinNode(LinkedList **head) {LinkedList *tmpHead = createLinkedList(-1);tmpHead->next = *head;LinkedList *cur = tmpHead->next;LinkedList *prev = tmpHead;LinkedList *prevMin = tmpHead;LinkedList *minNode = cur;while (cur != NULL) {if (cur->data < minNode->data) {prevMin = prev;minNode = cur;}prev = cur;cur = cur->next;}prevMin->next = minNode->next;*head = tmpHead->next;
}

运行

int main() {LinkedList *head1 = NULL;insertAtTail(&head1, 1);insertAtTail(&head1, 3);insertAtTail(&head1, 8);insertAtTail(&head1, 99);printAll(head1);deleteMinNode(&head1);printAll(head1);return 0;
}

int main() {LinkedList *head1 = NULL;insertAtTail(&head1, 88);insertAtTail(&head1, 3);insertAtTail(&head1, 8);insertAtTail(&head1, 99);printAll(head1);deleteMinNode(&head1);printAll(head1);return 0;
}

打印倒数第k个元素

利用快慢指针法，快指针先走k步；然后循环，快指针走到尾，慢指针刚好指向倒数第k个结点

void printLastKNode(LinkedList *head, int k) {LinkedList *current = head;LinkedList *slow = head;LinkedList *fast = NULL;while (k-->0) {fast = current->next;current = current->next;}while (fast != NULL) {slow = slow->next;fast = fast->next;}printf("--->%d", slow->data);
}

运行

int main() {LinkedList *head1 = NULL;insertAtTail(&head1, 88);insertAtTail(&head1, 3);insertAtTail(&head1, 8);insertAtTail(&head1, 99);printAll(head1);printLastKNode(head1, 1);printf("\n");printLastKNode(head1, 2);printf("\n");printLastKNode(head1, 4);return 0;
}

循环右移k个位置

将循环右移k个位置，例如k=2， 123456变为：561234
先找到分割结点的前一个结点，此时将链表断为两段，即：1–2–3–4–NULL 和 5–6–NULL
然后将后半段next指向前半段即可。

void rightCycleKNode(LinkedList **head, int k) {int length = 1;LinkedList *backNode = *head;while (backNode->next != NULL) {length++;backNode = backNode->next;}// now back is point at tail: 5--6--NULLint preK = length - k - 1;LinkedList *front = *head;LinkedList *newHead = NULL;// 找到分割结点的前一个结点，此时也可以快慢指针，先让快指针先走k步，然后快指针指向尾结点时，慢指针刚好指向分割结点的前一个结点while (preK-- > 0) {front = front->next;}// after loop, now node is point at 4newHead = front->next;  // 此时new： 5--6--NULLfront->next = NULL; // cut front line: 1--2--3--4--NULL（head此时和front一样，只是指向的结点不一样）backNode->next = *head;  // 把1234接到6的后边，6--1--2*head = newHead;
}

运行

int main() {LinkedList *head1 = NULL;insertAtTail(&head1, 1);insertAtTail(&head1, 2);insertAtTail(&head1, 3);insertAtTail(&head1, 4);insertAtTail(&head1, 5);insertAtTail(&head1, 6);printAll(head1);rightCycleKNode(&head1, 2);printAll(head1);return 0;
}

判断链表是否有环

采用快慢指针法，快指针每次走两步，慢指针每次走一步，如果存在环，快指针最终会沿着环与慢指针相遇（类似于跑步中的套圈）

运行

int main() {LinkedList *head1 = NULL;insertAtTail(&head1, 1);insertAtTail(&head1, 2);insertAtTail(&head1, 3);hasCycle(head1);head1->next->next->next = head1;hasCycle(head1);return 0;
}