【数据结构】链表（leetcode）

[① 203.移除链表元素](#① 203.移除链表元素)

[② 206.反转链表](#② 206.反转链表)

[③ 876.链表的中间节点](#③ 876.链表的中间节点)

[④ 返回倒数第k个节点（面试题）](#④ 返回倒数第k个节点（面试题）)

[⑤ 21.合并两个有序链表](#⑤ 21.合并两个有序链表)

[⑥ 160.相交链表](#⑥ 160.相交链表)

[⑦ 138.随机链表的复制（深拷贝）](#⑦ 138.随机链表的复制（深拷贝）)

① 203.移除链表元素

cpp 复制代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
 typedef struct ListNode ListNode;
struct ListNode* removeElements(struct ListNode* head, int val) {
    ListNode*newHead = NULL, *newTail = NULL;
    ListNode*pcur = head;
    while(pcur != NULL)
    {
        if(pcur->val != val)
        {
            if(newHead == NULL)
            {
                //将新链表的头尾指针指向原链表头节点
                newHead = newTail = pcur;
            }
            else
            {
                newTail->next = pcur;
                newTail = newTail->next;
            }
        }
        pcur = pcur->next;
    }

    if(newTail != NULL)
    {
        newTail->next = NULL;
    }                                                                        
    return newHead;
}

② 206.反转链表

cpp 复制代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

struct ListNode* reverseList(struct ListNode* head) {
    struct ListNode* prev = NULL;//哨兵位
    struct ListNode* cur = head;//头节点
    while (cur) 
    {
        // 哨兵位（prev）  节点1（cur）  节点2（cur->next）
        struct ListNode* next = cur->next;//创建一个中间节点
        //开始改变链表的方向
        cur->next = prev;//节点2先指向节点1的前一个节点
        prev = cur;//哨兵位往后移动
        cur = next;//节点1向后移动
    }
    return prev;
}

③ 876.链表的中间节点

cpp 复制代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

struct ListNode* middleNode(struct ListNode* head) {
    //慢指针（一次走一步）
    struct ListNode* slow = head;
    //快指针（一次走两步）
    struct ListNode* fast = head;
    while(fast && fast->next){
        slow = slow->next;
        fast = fast->next->next;
    }
    return slow; 
}

④ 返回倒数第k个节点（面试题）

cpp 复制代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
int kthToLast(struct ListNode* head, int k) {
    struct  ListNode *fast = head, *slow = head;
    //本题采用的是相对法：
    //      fast先运动k个节点
    //      假设该链表的节点个数是 m+k 个
    //      则先走k个节点，剩下fast指针到null指针时，即走了m个节点
    //      此时，slow指针就剩余k个节点
    while(k--){
        fast = fast->next;
    }
    while(fast != NULL){
        slow = slow->next;
        fast = fast->next;   
    }
    return slow->val;
}

⑤ 21.合并两个有序链表

cpp 复制代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) {
     if(list1 == NULL)
     {
        return list2;
    }if(list2 == NULL)
    {
        return list1;
    }

    struct ListNode* l1 = list1;
    struct ListNode* l2 = list2;
    struct ListNode* newHead, *newTail;                                                             
    newHead = newTail = NULL;

    while(l1  &&  l2)
    {
        //比大小
        if(l1->val < l2->val)
        {
           if(newHead == NULL)
           {
            newHead = newTail = l1;
           }
           else
           {
            newTail->next = l1;
            newTail = newTail->next;
           }
           l1 = l1->next;
        }
        else
        {
            if(newHead == NULL)
            {
                newHead = newTail = l2;
            }else
            {
                newTail->next = l2;
                newTail = newTail->next;
            }
            l2 = l2->next;
        }
    }
    if(l1)
    {
        newTail->next = l1;
    }
    if(l2)
    {
        newTail->next = l2;
    }
    return newHead;
}

⑥ 160.相交链表

cpp 复制代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *getIntersectionNode(struct ListNode *headA, struct ListNode *headB) {
    struct ListNode* curA = headA,  *curB = headB;
    int lenA = 1, lenB = 1;
    while(curA->next){
        curA = curA->next;
        ++lenA;
    }
    while(curB->next){
        curB = curB->next;
        ++lenB;
    }
    if(curA != curB){
        return NULL;
    }

    //假设法
    int gap = abs(lenA - lenB);
    struct ListNode* longList = headA, *shortList = headB;
    if(lenB > lenA){
        longList = headB;
        shortList = headA;
    }`
    while(gap--){
        longList = longList->next;
    }
    while(longList != shortList){
        longList = longList->next;
        shortList = shortList->next;
    }
    return longList;
    
}

⑦ 138.随机链表的复制（深拷贝）

cpp 复制代码

/**
 * Definition for a Node.
 * struct Node {
 *     int val;
 *     struct Node *next;
 *     struct Node *random;
 * };
 */

struct Node* copyRandomList(struct Node* head) {
	struct Node* cur = head;
    while(cur){
        struct Node * copy = (struct Node*)malloc(sizeof(struct Node));
        //赋值
        copy->val = cur->val;
     
        //将copy链表插入原链表中
        copy->next = cur->next;
        cur->next = copy;
        //cur向后移动
        cur = copy->next;
    }
    //将copy链表中random指针的指向与原链表中的指针对调
    cur = head;
    while(cur){
        struct Node* copy = cur->next;
        if(cur->random == NULL){
            copy->random = NULL;
        }
        else{
            copy->random = cur->random->next;
        }
        cur = copy->next;
    }

    cur = head;
    struct Node* copyHead = NULL,* copyTail =NULL;
    while(cur){
        struct Node* copy = cur->next;

        if(copyTail == NULL)
        {
            copyHead = copyTail = copy;
        }else
        {
            copyTail->next = copy;
            copyTail = copyTail->next;
        }
        //cur向后移动
        cur = copy->next;
    }
    return copyHead;
}