21.Merge Two Sorted Lists

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LinkedList, Easy

Question

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:

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

Answer

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class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode res = new ListNode(0);
        ListNode pre = res;
        while(l1!=null && l2!=null){
            if(l1.val < l2.val){
                pre.next = l1;
                l1 = l1.next;
            }
            else{
                pre.next = l2;
                l2 = l2.next;
            }
            pre = pre.next;
        }
        if(l1 == null)
            pre.next = l2;
        if(l2 == null)
            pre.next = l1;
        return res.next; // first node is 0, skim it
    }
}

Running time: O(n), 16ms