21. 合并两个有序链表

Posted 2022-03-07 Ston.V

1.Description

将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

2.Example

 

3.My Code

直接合并

/**
 * 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) 
        ListNode *p=list1,*q=list2;
        ListNode *head = new ListNode(-1);
        ListNode * r = head;
        while(p && q)
            if(p->val <= q->val)
                r ->next = new ListNode(p->val);
                p=p->next;
            else if(p->val > q->val)
                r ->next = new ListNode(q->val);
                q=q->next;
            
            r = r->next;
        
        if(p)r->next = p;
        if(q)r->next = q;
        return head->next;
        
    

4.Code(递归解法)

class Solution 
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) 
        if (l1 == nullptr) 
            return l2;
         else if (l2 == nullptr) 
            return l1;
         else if (l1->val < l2->val) 
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
         else 
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        
    
;