单向循环链表：
如果把单链表的最后一个节点的指针指向链表头部，而不是指向NULL，那么就构成了一个单向循环链表。在单向链表中，头指针是相当重要的，因为单向链表的操作都需要头指针，所以如果头指针丢失或者破坏，那么整个链表都会遗失，并且浪费链表内存空间，因此我们引入了单向循环链表这种数据结构。
注意：
循环链表中没有NULL指针。涉及遍历操作时，其终止条件就不再是像非循环链表那样判别p或p->next是否为空，而是判别它们是否等于某一指定指针，如头指针或尾指针等。在单链表中，从一已知结点出发，只能访问到该结点及其后续结点，无法找到该结点之前的其它结点。而在单循环链表中，从任一结点出发都可访问到表中所有结点，这一优点某些运算在单循环链表上易于实现。
1 class Node(): 2 '''节点类''' 3 def __init__(self, elem): 4 '''节点的两个参数，本身值elem以及指向下一个节点的地址next''' 5 self.elem = elem 6 self.next = None 7 # self.prev = None 8 9 10 class SingCycleLinkList(): 11 '''双链表''' 12 def __init__(self, node = None): 13 self.__head = node 14 if node: 15 node.next = node 16 17 def sing_append(self, elem): 18 '''链表尾部添加元素''' 19 node = Node(elem) 20 if self.is_empty(): 21 self.__head = node 22 node.next = node 23 else: 24 cur = self.__head 25 while cur.next != self.__head: 26 cur = cur.next 27 cur.next = node 28 node.next = self.__head 30 31 def is_empty(self): 32 '''判断链表是否为空，第一次传入数据以后self.__head将永远指向node1对象''' 33 return self.__head is None 34 35 def sing_length(self): 36 '''链表长度''' 37 if self.is_empty(): 38 return 0 39 cur = self.__head 40 # 将第一个元素的node实例赋值给cur， 41 count = 1 42 while cur.next != self.__head: 43 count+=1 44 cur = cur.next 45 return count 46 47 def sing_travel(self): 48 '''遍历整个列表''' 49 if self.is_empty(): 50 return 51 cur = self.__head 52 # 即将node1对象赋值给cur 53 while cur.next != self.__head: 54 print(cur.elem) 55 cur = cur.next 56 # 退出循环后最后一个节点不会再循环体内打印出来 57 print(cur.elem) 58 59 def sing_add(self, item): 60 node = Node(item) 61 if self.is_empty(): 62 self.__head = node 63 node.next = node 64 return 65 cur = self.__head 66 while cur.next != self.__head: 67 cur = cur.next 68 node.next = self.__head 69 self.__head = node 70 cur.next = node 71 72 def sint_insert(self, pos, item): 73 if pos <= 0: 74 self.sing_add(item) 75 elif pos > self.sing_length()-1: 76 self.sing_append(item) 77 else: 78 cur = self.__head 79 count = 0 80 while count < pos: 81 count += 1 82 cur = cur.next 83 node = Node(item) 84 node.next = cur.next 85 cur.next = node 86 88 def sing_search(self,item): 89 if self.is_empty(): 90 return False 91 cur = self.__head 92 while cur.next != self.__head: 93 if item == cur.elem: 94 return True 95 else: 96 cur = cur.next 97 # 退出循环是判断尾结点 98 if cur.elem == item: 99 return True 100 return False 101 102 def sing_remove(self, item): 103 if self.is_empty(): 104 return 105 cur = self.__head 106 pre = None 107 while cur.next != self.__head: 108 if item == cur.elem: 109 # 判断是否为头节点 110 if cur == self.__head: 111 rear = self.__head 112 while rear.next != self.__head: 113 rear = rear.next 114 self.__head = cur.next 115 rear.next = self.__head 116 else: 117 # 中间节点 118 pre.next = cur.next 119 return 120 else: 121 pre = cur 122 cur = cur.next 123 if cur.elem == item: 124 # 尾结点 125 if cur == self.__head: 126 self.__head = None 127 else: 128 pre.next = cur.next 129 130 131 if __name__ == "__main__": 132 ll = SingCycleLinkList() 133 ll.sing_append(1) 134 ll.sing_append(2) 135 # ll.sing_add(8) 136 ll.sing_append(3) 137 ll.sint_insert(-1,5) 138 ll.sing_travel() 139 print(ll.sing_search(5)) 140 ll.sing_remove(5) 141 ll.sing_travel()