L6.【LeetCode笔记】合并两个有序链表

1.题目

2.自解

一个容易想到的解法:取小的尾插(开一个新的链表)

不加思索写出以下代码:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) 
{
    struct ListNode* cur1=list1;
    struct ListNode* cur2=list2;

    if (list1==NULL)
        return list2;
    if (list2==NULL)
        return list1;
    
    struct newListNode
    {
        int new_val;
        struct ListNode* new_next;
    };
    struct newListNode* new_next=NULL;
    struct newListNode* newhead=NULL;
    struct newListNode* m_m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
    newhead=m_m_new;
    newhead->new_next=NULL;
    struct newListNode* new_cur=newhead;
    while(cur1!=NULL && cur2!=NULL)
    {
        if (cur1==NULL)
        {
            new_cur->new_val=cur2->val;
            cur2=cur2->next;
            //分配新结点的空间
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            new_cur=m_new;
            continue;
        }
        if (cur2==NULL)
        {
            new_cur->new_val=cur1->val;
            cur1=cur1->next;
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            new_cur=m_new;
            continue;
        }
        if (cur1->val<=cur2->val)
        {
            new_cur->new_val=cur1->val;
            cur1=cur1->next;
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            new_cur=m_new;
        }
        else
        {
            new_cur->new_val=cur2->val;
            cur2=cur2->next;
            //分配新结点的空间
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            new_cur=m_new;
        }
    }
    new_cur->new_next=NULL;
    new_cur=NULL;
    return newhead;
}
 

运行时出现问题

发现while循环的条件写错了!!

应该改成

while(!(cur1==NULL && cur2==NULL))

完整代码

struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) 
{
    struct ListNode* cur1=list1;
    struct ListNode* cur2=list2;

    if (list1==NULL)
        return list2;
    if (list2==NULL)
        return list1;
    
    struct newListNode
    {
        int new_val;
        struct ListNode* new_next;
    };
    struct newListNode* new_next=NULL;
    struct newListNode* newhead=NULL;
    struct newListNode* m_m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
    newhead=m_m_new;
    newhead->new_next=NULL;
    struct newListNode* new_cur=newhead;
    struct newListNode* before_new_cur=NULL;

    while(!(cur1==NULL && cur2==NULL))
    {
        if (cur1==NULL)
        {
            new_cur->new_val=cur2->val;
            cur2=cur2->next;
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            before_new_cur=new_cur;
            new_cur=m_new;
            new_cur->new_next=NULL;
            continue;
        }
        if (cur2==NULL)
        {
            new_cur->new_val=cur1->val;
            cur1=cur1->next;
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            before_new_cur=new_cur;
            new_cur=m_new;
            continue;
        }
        
        if (cur1->val<=cur2->val)
        {
            new_cur->new_val=cur1->val;
            cur1=cur1->next;
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            new_cur=m_new;
        }
        else
        {
            new_cur->new_val=cur2->val;
            cur2=cur2->next;           
            struct newListNode* m_new=(struct newListNode*)malloc(sizeof(struct newListNode));
            new_cur->new_next=m_new;
            new_cur=m_new;
        }
    }
   before_new_cur->new_next=NULL;
   return newhead;
}
 

这里有一个潜在的问题:内存泄漏

应该在before_new_cur->new_next=NULL;下方加上free(new_next);

提交结果

3.其他解法

方法1:取小的尾插(不开新链表)

struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) 
{
    struct ListNode* cur1=list1;
    struct ListNode* cur2=list2;
    struct ListNode* head=NULL;
    struct ListNode* tail=NULL;

    if (list1==NULL)
        return list2;
    if (list2==NULL)
        return list1;

    while (cur1 && cur2)
    {
        if (cur1->val<cur2->val)
        {
            if (head==NULL)
            {
                head=tail=cur1;
            }
            else
            {
                tail->next=cur1;
                tail=tail->next;
            }
            cur1=cur1->next;
        }
        else
        {
             if (head==NULL)
            {
                head=tail=cur2;
            }
            else
            {
                tail->next=cur2;
                tail=tail->next;
            }
            cur2=cur2->next;           
        }
    }
    
    if(cur1)
        tail->next=cur1;
    if(cur2)
        tail->next=cur2;
        
    return head;
}

分析

尾插要有尾指针tail(这样不用频繁找尾),同时要有指向头节点的指针head用于返回

cur1->val<cur2->val和cur1->val>=cur2->val操作方式是类似的

当只有一个节点时,head==tail(即头==尾)

当cur1为NULL时,直接将剩余链表全部移动(head=tail=cur2;或head=tail=cur1;)

方法2:引入带哨兵位的头结点guard

在方法1代码的基础上修改

struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) 
{
    struct ListNode* cur1=list1;
    struct ListNode* cur2=list2;
    struct ListNode* guard=NULL;
    struct ListNode* tail=NULL;
    guard=tail=(struct ListNode*)malloc(sizeof(struct ListNode));
    tail->next=NULL;
    if (list1==NULL)
        return list2;
    if (list2==NULL)
        return list1;
 
    while (cur1 && cur2)
    {
        if (cur1->val<cur2->val)
        {
            tail->next=cur1;
            tail=tail->next;
            cur1=cur1->next;
        }
        else
        {
            tail->next=cur2;
            tail=tail->next;
            cur2=cur2->next;           
        }
    }
    
    if(cur1)
        tail->next=cur1;
    if(cur2)
        tail->next=cur2;
        
    return guard->next;
}

虽然提交后能通过,但是有一个显然的问题:内存泄漏,要把头结点的空间释放掉

因此return guard->next;改为

    struct ListNode* head=guard->next;
    free(guard); 
    return head;