【LeetCode】21 - Merge Two Sorted Lists

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.

Tags： Linked List

Solution 1 :  create a new list

/**
 * 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) {
        ListNode *newNode=new ListNode(-1);
        ListNode *ret=newNode;
        while(l1 && l2){
            if(l1->val <= l2->val){
                ret->next=l1;
                l1=l1->next;
            }else{
                ret->next=l2;
                l2=l2->next;
            }
            ret=ret->next;
        }
        if(l1)
            ret->next=l1;
        else
            ret->next=l2; 
        return newNode->next;
    }
};

 

Solution 2 : recursion

    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == NULL) return l2;
        if (l2 == NULL) return l1;
    
        ListNode *ret = NULL;
    
        if (l1->val <= l2->val){
            ret = l1;
            ret->next = mergeTwoLists(l1->next, l2);
        }
        else{    
            ret = l2;
            ret->next = mergeTwoLists(l1, l2->next);
        }    
        return ret;
    }