用c/c++写一个程序将两个升序链表合并成一个升序链表

好的，这是一个比较常见的链表问题，可以使用递归或迭代的方式来解决。以下是一个使用迭代的示例代码：

#include <stdio.h>
#include <stdlib.h>

struct ListNode {
    int val;
    struct ListNode *next;
};

struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode dummy;
    struct ListNode *tail = &dummy;
    
    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 dummy.next;
}

int main() {
    struct ListNode *l1 = malloc(sizeof(struct ListNode));
    l1->val = 1;
    l1->next = malloc(sizeof(struct ListNode));
    l1->next->val = 2;
    l1->next->next = malloc(sizeof(struct ListNode));
    l1->next->next->val = 4;
    l1->next->next->next = NULL;
    
    struct ListNode *l2 = malloc(sizeof(struct ListNode));
    l2->val = 1;
    l2->next = malloc(sizeof(struct ListNode));
    l2->next->val = 3;
    l2->next->next = malloc(sizeof(struct ListNode));
    l2->next->next->val = 4;
    l2->next->next->next = NULL;
    
    struct ListNode *merged = mergeTwoLists(l1, l2);
    
    while (merged) {
        printf("%d ", merged->val);
        merged = merged->next;
    }
    
    return 0;
}

这个程序将两个升序链表合并成一个升序链表，并输出结果：1 1 2 3 4 4。