leetcode 30: 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;
*     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(!l2) return l1;
if(!l1) return l2;

ListNode* head = NULL;
ListNode** pre = &head;

while( l1 && l2){

if(l1->val < l2->val){
*pre = l1;
l1 = l1->next;
} else {
*pre = l2;
l2 = l2->next;
}

pre = &((*pre)->next);
}

if(l1) *pre = l1;
if(l2) *pre = l2;

return head;

}
};

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
// Start typing your Java solution below
// DO NOT write main() function

ListNode head = new ListNode(0); //dummy
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; ///don't forget this. always check loop terminator conditions.
}

if(l1!=null) p.next = l1;
else if(l2!=null) p.next = l2;

return head.next;

}
}