[leetcode]: 21. Merge Two Sorted Lists

1.题目

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.
合并两个有序的列表

2.分析

遍历两个列表，每次将较小的节点加入到合并列表的尾部。

3.代码

class Solution {
public:
    //递归
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == NULL)
            return l2;
        if (l2 == NULL)
            return l1;
        ListNode* node = NULL;
        if (l1->val < l2->val){
            node = l1;
            node->next = mergeTwoLists(l1->next, l2);
        }
        else {
            node = l2;
            node->next = mergeTwoLists(l1, l2->next);
        }
        return node;
    }
    //迭代
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == NULL)
            return l2;
        if (l2 == NULL)
            return l1;
        ListNode* head = new ListNode(0);
        ListNode* tail = head;
        while (l1&&l2) {
            if (l1->val < l2->val){
                tail->next = l1;
                l1=l1->next;
            }
            else{
                tail->next = l2;
                l2=l2->next;
            }
            tail = tail->next;
        }
        tail->next = l1 ? l1 : l2;
        return head->next;
    }
};