链表问题---合并两个有序的单链表

【题目】

　　给定两个有序单链表的头节点head1和head2，请合并两个有序链表，合并后的链表依然有序，并返回合并后链表的头节点。

【代码实现】

#python3.5def merge(head1, head2):    if head1 == None or head2 == None:        return head1 if head2 == None else head2    head = head1 if head1.val < head2.val else head2    cur1 = head1 if head == head1 else head2    cur2 = head1 if head == head2 else head2    pre = None    while cur1 != None and cur2 != None:        if cur1.val <= cur2.val:            pre = cur1            cur1 = cur1.next        else:            next = cur2.next            pre.next = cur2            cur2.next = cur1            pre = cur2            cur2 = next    pre.next = cur1 if cur2 == None else cur2    return head