[每日 LeetCode] 21. Merge Two Sorted Lists

Description:

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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

思路：本题要求合并两个有序的链表。同样有迭代和递归的方法。

迭代方法：以 l1 链表为主，依次遍历 l2，将 l2 的节点合并到 l1 中。

递归方法：每次返回 l1 和 l2 中较小的节点。

C++ 代码（迭代）

/**
 * 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 == nullptr)
            return l2;
        if (l2 == nullptr)
            return l1;
        if (l1->val > l2->val)
            swap(l1,l2);
        ListNode* head = l1, *tail = l1;
        l1 = l1->next;
        while(l1 && l2){
            if (l1->val < l2->val){
                tail->next = l1;
                tail = l1;
                l1 = l1->next;
            }
            else{
                tail->next = l2;
                tail = l2;
                l2 = l2->next;
            }
        }
        if (l1 != nullptr)
            tail->next = l1;
        if (l2 != nullptr)
            tail->next = l2;
        return head;
    }
};

运行时间：8ms

运行内存：9M

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

运行时间：12ms

运行时间：9M