LeetCode之Merge Two Sorted Lists

http://oj.leetcode.com/problems/merge-two-sorted-lists/

合并两个有序链表，重点在于  头指针的建立 （新建一个头指针省时省力）

/**
 * 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) {
        ListNode *root = new ListNode(-1);
        ListNode *now_pos = root;
        
        while(l1 && l2) {
            if(l1->val > l2->val) {
                now_pos ->next = l2;
                l2 = l2->next;
            } else {
                now_pos ->next = l1;
                l1 = l1->next;
            }
            now_pos = now_pos->next;
        }
        
        if(l1) {
            now_pos->next = l1;
        } else if(l2) {
            now_pos->next = l2;
        }
        
        return root->next;
    }
};