LeetCode 21 Merge Two Sorted Lists 链表合并

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Solution

先新建一个头节点：

点击查看代码

        ListNode newhead(0);
        ListNode* p = &newhead;

然后利用双指针即可：

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        // create a new headnode
        ListNode newhead(0);
        ListNode* p = &newhead;
        
        if(!list1 && !list2) return nullptr;
        else if(!list1) return list2;
        else if(!list2) return list1;
        else{
            while(list1||list2){
                if(!list1){
                    while(list2){
                        p->next = list2;
                        p = p->next;
                        list2 = list2->next;
                    }
                    break;
                }
                
                if(!list2){
                    while(list1){
                        p->next = list1;
                        p = p->next;
                        list1 = list1->next;
                    }
                    break;
                }
                
                if(list1->val <= list2->val){
                    p->next = list1;
                    p = p->next;
                    list1 = list1->next;
                }
                else {
                    p->next = list2;
                    p = p->next;
                    list2 = list2->next;
                }
            }
        }
        return newhead.next;
    }
};