c++ - 无法使用辅助链表合并排序链表

Question

请给出问题的简单解决方案。我使用了像 mergesort 这样的算法，但我无法返回我创建的辅助链表的头部。我看过其他关于堆栈溢出的例子。但是我想知道我的代码哪里有问题。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode* Solution::mergeTwoLists(ListNode* A, ListNode* B) {

    ListNode* head;
    ListNode* root;

    ListNode* H1 = A;
    ListNode* H2 = B;
    int flag = 0;
    while (H1 != NULL && H2 != NULL){
        if(H1->val < H2->val){
            root = new ListNode(H1->val);
            //cout << root->val << " ";
            if (flag = 0){
                 head = root;
                flag = 1;
            }

            //root->next = el;
            root = root->next;
            H1 = H1->next;
        }else{
            root = new ListNode(H2->val);
            if (flag = 0){
               head  =root;
                flag = 1;
            }
            //cout << root->val << " ";
            //root->next = el;
            root = root->next;
            H2 = H2->next;
        }
    }
    while (H2 != NULL){

        root = new ListNode(H2->val);
        //cout << root->val << " ";
        //root->next = el;
        root = root->next;
        H2 = H2->next;
    }
    while (H1 != NULL){
        root = new ListNode(H1->val);
        //cout << root->val << " ";
        //root->next = el;
        root = root->next;
        H1 = H1->next;
    }

     ListNode *start=head;
        while(start)
          {
            cout<<start->val<<" ";
            start=start->next;
          }


    return head;
}

我用 cout 知道顺序，它给出了正确的顺序。我在这里遗漏了一些东西。列表都不为 NULL

Answer 1

在您的代码中发现了两个问题。首先，应该在两个地方将等于运算符更改为 bool 值:

if (flag = 0){

应该是

if (flag == 0){

那么，遍历这两个链表的时候要保留一个尾节点。

我转换了这里的代码(应用最小的更改)，它有效:

ListNode* mergeTwoLists(ListNode* A, ListNode* B) {

    ListNode* head;
    ListNode* tail; //<-- a tail is introduced
    ListNode* root;

    ListNode* H1 = A;
    ListNode* H2 = B;
    int flag = 0;
    while (H1 != NULL && H2 != NULL){
        if(H1->val < H2->val){
            root = new ListNode(H1->val);
            //cout << root->val << " ";
            if (flag == 0){ //<-- fixed
                 head = root;
                 tail=head;
                flag = 1;
            }
            else
            {
                tail->next=root;
                tail = root;
            }


            //root->next = el;
            //root = root->next;
            H1 = H1->next;
        }else{
            root = new ListNode(H2->val);
            if (flag == 0){  //<-- fixed
               head  =root;
               tail=head;
                flag = 1;
            }
            else
            {
                tail->next=root;
                tail = root;
            }
            //cout << root->val << " ";
            //root->next = el;
           // root = root->next;
            H2 = H2->next;
        }
    }
    while (H2 != NULL){

        root = new ListNode(H2->val);
        //cout << root->val << " ";
        //root->next = el;
        tail->next=root;
        tail=root;
       // root = root->next;
        H2 = H2->next;
    }
    while (H1 != NULL){
        root = new ListNode(H1->val);
        //cout << root->val << " ";
        //root->next = el;
        tail->next=root;
        tail=root;
        //root = root->next;
        H1 = H1->next;
    }

     ListNode *start=head;
        while(start)
          {
            cout<<start->val<<" ";
            start=start->next;
          }


    return head;
}