leetcode (21) - Merge Two Sorted Lists

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.

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next; * }; */struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {    if(!l1 && !l2) return NULL;    if(l1 && !l2) return l1;    if(!l1 && l2) return l2;        struct ListNode* p1=l1;    struct ListNode* p2=l2;    struct ListNode* ret_head=malloc(sizeof(struct ListNode));    struct ListNode* ret_p=ret_head;    ret_p->next=NULL;    if(p1->val > p2->val){        ret_p->val=p2->val;        p2=p2->next;    } else {        ret_p->val=p1->val;        p1=p1->next;    }        struct ListNode* new_node;    while(p1 && p2){        new_node=malloc(sizeof(struct ListNode));        new_node->next=NULL;                if(p1->val > p2->val){            new_node->val=p2->val;            p2=p2->next;        } else {            new_node->val=p1->val;            p1=p1->next;        }                ret_p->next=new_node;        ret_p=new_node;            }        while(p1){        new_node=malloc(sizeof(struct ListNode));        new_node->next=NULL;        new_node->val=p1->val;        p1=p1->next;        ret_p->next=new_node;        ret_p=new_node;    }        while(p2){        new_node=malloc(sizeof(struct ListNode));        new_node->next=NULL;        new_node->val=p2->val;        p2=p2->next;        ret_p->next=new_node;        ret_p=new_node;    }        return ret_head;}