Leetcode 21. Merge Two Sorted Lists

21. Merge Two Sorted Lists

• Total Accepted: 138746
• Total Submissions: 383218
• Difficulty: Easy

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

思路：递归或者迭代添加节点。

代码：

递归：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(!l1) return l2;
        if(!l2) return l1;
        if(l1->val<l2->val){
            l1->next=mergeTwoLists(l1->next,l2);
            return l1;
        }
        l2->next=mergeTwoLists(l1,l2->next);
        return l2;
    }
};

迭代：

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head=new ListNode(-1);
        ListNode* end=head;
        while(l1&&l2){
            if(l1->val<l2->val){
                end->next=l1;
                l1=l1->next;
            }
            else{
                end->next=l2;
                l2=l2->next;
            }
            end=end->next;
        }
        if(l1){
            end->next=l1;
        }
        else{
            end->next=l2;
        }
        return head->next;
    }
};