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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4


合并两个有序单链表


C++（13ms）：

/**
 * 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* head = NULL ;
        if (l1->val <= l2->val){
            head = l1 ;
            l1 = l1->next ;
        }else{
            head = l2 ;
            l2 = l2->next ;
        }
        ListNode* p = head ;
        while(l1 != NULL && l2 != NULL){
            if (l1->val <= l2->val){
                p->next = l1 ;
                l1 = l1->next ;
            }else{
                p->next = l2 ;
                l2 = l2->next ;
            }
            p = p->next ;
        }
        p->next = l1 ? l1 : l2 ;
        return head ;
    }
};