#21 Merge Two Sorted Lists

題目鏈接：https://leetcode.com/problems/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) {
    struct ListNode* head;
    struct ListNode* p;
    if(l1 == NULL)
        return l2;
    if(l2 == NULL)
        return l1;
    if(l1->val < l2->val) {
        head = l1;
        l1 = l1->next;
    }
    else {
        head = l2;
        l2 = l2->next;
    }
    p = head;
    while(l1 && l2) {
        if(l1->val < l2->val) {
            p->next = l1;
            l1 = l1->next;
            p = p->next;
        }
        else {
            p->next = l2;
            l2 = l2->next;
            p = p->next;
        }
    }
    if(l1)
        p->next = l1;
    if(l2)
        p->next = l2;
    return head;
}