0. 题目

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:

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

1. C++版本

 ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* newHead = new ListNode(-1);
        ListNode* curr = newHead;
        ListNode* p = l1; ListNode* q = l2;
        while (p && q) {
            if (p->val <= q->val){
                curr->next = p;
                curr = curr->next;
                p = p->next;
            }
            else {
                curr->next = q;
                curr = curr->next;
                q = q->next;
            }
        }
        while (p) {curr->next = p; curr = curr->next; p = p->next;}
        while (q) {curr->next = q; curr = curr->next; q = q->next;}
        return newHead->next;
    }

优化版本：引自https://leetcode.com/problems/merge-two-sorted-lists/discuss/9714/14-line-clean-C%2B%2B-Solution

 ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* newHead = new ListNode(-1);
        ListNode* curr = newHead;
        while (l1 && l2) {
            if (l1->val <= l2->val){
                curr->next = l1;
                l1 = l1->next;
            }
            else {
                curr->next = l2;
                l2 = l2->next;
            }
            curr = curr->next;
        }
        curr->next = l1 ? l1 : l2;
        return newHead->next;
    }

2. python版本

  def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        newHead = ListNode(-1)
        curr, p , q = newHead, l1, l2
        while p and q:
            if p.val <= q.val:
                curr.next = p
                curr = curr.next
                p = p.next
            else:
                curr.next = q
                curr = curr.next
                q = q.next
        while p:
                curr.next = p
                curr = curr.next
                p = p.next
        while q:
                curr.next = q
                curr = curr.next
                q = q.next
        return newHead.next

看下比较更加pythonic的解法，引自 https://leetcode.com/problems/merge-two-sorted-lists/discuss/9735/Python-solutions-(iteratively-recursively-iteratively-in-place).

    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        dummy = curr = ListNode(-1)
        while l1 and l2:
            if l1.val <= l2.val:
                curr.next = l1
                l1 = l1.next
            else:
                curr.next = l2
                l2 = l2.next
            curr = curr.next
        curr.next = l1 or l2
        return dummy.next