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.

分析

建立dummy和prev，相等，然后prev一个个传下去。

iteration

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode prev = dummy;

        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                prev.next = l1;
                l1 = l1.next;
            }else{
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }

        if(l1 != null){
            prev.next = l1;
        }
        if(l2 != null){
            prev.next = l2;
        }

        return dummy.next;
    }
}

Recursive

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;

       if(l1.val < l2.val){
           l1.next = mergeTwoLists(l1.next, l2);
           return l1;
       }else{
           l2.next = mergeTwoLists(l1, l2.next);
           return l2;
       }
    }
}