合并两个有序链表

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        prev1 = ListNode(0)
        prev2 = ListNode(0)
        prev1.next = l1
        prev2.next = l2
        head = prev1
        while prev1.next and prev2.next:
            if prev1.next.val <= prev2.next.val:
                prev1 = prev1.next
            else:
                prev1.next,prev2.next = prev2.next,prev1.next
                prev1 = prev1.next
        if prev2.next:
            prev1.next = prev2.next
        return head.next