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合并两个排序链表

合并两个排序链表

一、题目

输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。

leetcode：https://leetcode.cn/problems/he-bing-liang-ge-pai-xu-de-lian-biao-lcof/

二、思路

简单，注意开始的指向和结束的指向

三、code

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/**
 * 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 == NULL) return l2;
        if (l2 == NULL) return l1;
        ListNode* res = NULL;
        if (l1->val < l2->val) {
            res = l1;
            l1 = l1->next;
        } else {
            res = l2;
            l2 = l2->next;
        }
        ListNode* tmp = res;
        for (; l1 != NULL && l2 != NULL;) {
            if (l1->val < l2->val) {
                tmp->next = l1;
                l1 = l1->next;
            } else {
                tmp->next = l2;
                l2 = l2->next;
            }
            tmp = tmp->next;
        }
        for (; l1 != NULL;) {
            tmp->next = l1;
            tmp = tmp->next;
            l1 = l1->next;
        }
        for (; l2 != NULL;) {
            tmp->next = l2;
            tmp = tmp->next;
            l2 = l2->next;
        }
        return res;
    }
};