[LeetCode] 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.

维护一个新的链表，用两个指针指向两个链表，类似merge sot的比较。

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (l1 == NULL)
            return l2;
            
        if (l2 == NULL)
            return l1;
            
        ListNode *p = l1;
        ListNode *q = l2;
        ListNode *head = NULL;
        ListNode *curNode = NULL;
        
        while(p && q)
        {
            if (head == NULL)
            {
                if (p->val < q->val)
                {
                    head = curNode = p;
                    p = p->next;
                    curNode->next = NULL;
                }
                else
                {
                    head = curNode = q;
                    q = q->next;
                    curNode->next = NULL;
                }
            }
            else
            {
                if (p->val < q->val)
                {
                    curNode->next = p;
                    curNode = p;
                    p = p->next;
                    curNode->next = NULL;
                }
                else
                {
                    curNode->next = q;
                    curNode = q;
                    q = q->next;
                    curNode->next = NULL;
                }
            }
        }
        
        if (p)
            curNode->next = p;
        else if (q)
            curNode->next = q;
            
        return head;       
    }
};