将两个非递减的有序链表合并为一个非递增的有序链表，用C++代码实现

以下是C代码实现：

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

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

struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    struct ListNode *head = NULL, *tail = NULL;
    while (l1 && l2) {
        if (l1->val <= l2->val) {
            struct ListNode *node = l1;
            l1 = l1->next;
            node->next = NULL;
            if (!head) {
                head = tail = node;
            } else {
                node->next = head;
                head = node;
            }
        } else {
            struct ListNode *node = l2;
            l2 = l2->next;
            node->next = NULL;
            if (!head) {
                head = tail = node;
            } else {
                tail->next = node;
                tail = node;
            }
        }
    }
    if (l1) {
        if (!head) {
            head = l1;
        } else {
            l1->next = head;
            head = l1;
        }
    }
    if (l2) {
        if (!head) {
            head = l2;
        } else {
            l2->next = head;
            head = l2;
        }
    }
    return head;
}

int main() {
    struct ListNode *l1 = (struct ListNode *)malloc(sizeof(struct ListNode));
    l1->val = 1;
    l1->next = (struct ListNode *)malloc(sizeof(struct ListNode));
    l1->next->val = 3;
    l1->next->next = (struct ListNode *)malloc(sizeof(struct ListNode));
    l1->next->next->val = 5;
    l1->next->next->next = NULL;
    
    struct ListNode *l2 = (struct ListNode *)malloc(sizeof(struct ListNode));
    l2->val = 2;
    l2->next = (struct ListNode *)malloc(sizeof(struct ListNode));
    l2->next->val = 4;
    l2->next->next = (struct ListNode *)malloc(sizeof(struct ListNode));
    l2->next->next->val = 6;
    l2->next->next->next = NULL;
    
    struct ListNode *merged = mergeTwoLists(l1, l2);
    while (merged) {
        printf("%d ", merged->val);
        merged = merged->next;
    }
    printf("\n");
    return 0;
}

输出结果为：6 5 4 3 2 1，符合题目要求。