题目

Leetcode707. 设计链表

你可以选择使用单链表或者双链表，设计并实现自己的链表。
单链表中的节点应该具备两个属性：val 和 next 。val 是当前节点的值，next 是指向下一个节点的指针/引用。
如果是双向链表，则还需要属性 prev 以指示链表中的上一个节点。假设链表中的所有节点下标从 0 开始。

实现 MyLinkedList 类：

1. MyLinkedList() 初始化 MyLinkedList 对象。
2. int get(int index) 获取链表中下标为 index 的节点的值。如果下标无效，则返回 -1 。
3. void addAtHead(int val) 将一个值为 val 的节点插入到链表中第一个元素之前。在插入完成后，新节点会成为链表的第一个节点。
4. void addAtTail(int val) 将一个值为 val 的节点追加到链表中作为链表的最后一个元素。
5. void addAtIndex(int index, int val) 将一个值为 val 的节点插入到链表中下标为 index 的节点之前。
   如果 index 等于链表的长度，那么该节点会被追加到链表的末尾。如果 index 比长度更大，该节点将 不会插入 到链表中。
6. void deleteAtIndex(int index) 如果下标有效，则删除链表中下标为 index 的节点。

题解

class MyLinkedList:def __init__(self):self.val = Noneself.next = Noneself._length = 0  # 用于记录链表长度，用于移除、插入时检查前置条件self._last = self  # 记录末尾节点节点，用于快速尾插def get(self, index: int) -> int:"""获取第index个节点的值"""if index >= self._length:return -1res = selffor _ in range(index + 1):res = res.nextreturn res.valdef addAtHead(self, val: int) -> None:"""将值为val的节点插入链表表首[由于需要插入表首，self为虚拟节点，指向当前表首]"""tmp = MyLinkedList()tmp.val, tmp.next = val, self.nextself.next = tmpif self._length == 0:self._last = tmp  # 只有一个元素，将插入的节点标记为 _lastself._length += 1  # 更新链表长度def addAtTail(self, val: int) -> None:"""尾插"""tmp = MyLinkedList()tmp.val = val# 将节点插入链表尾端，并更新 _lastself._last.next = tmpself._last = tmpself._length += 1  # 更新链表长度def addAtIndex(self, index: int, val: int) -> None:"""在指定位置插入节点 """# 插入节点索引超过链表长度if index > self._length:returnif index == 0:# 首插self.addAtHead(val)elif index == self._length:# 尾插self.addAtTail(val)else:# 插入中间节点tmp = selffor _ in range(index):tmp = tmp.nexttmp.addAtHead(val)self._length += 1  # 更新链表长度def deleteAtIndex(self, index: int) -> None:"""删除指定位置节点"""# 插入节点索引超过链表长度if index >= self._length:returnelif index == 0:# 删除首节点self.next = self.next.nextelse:tmp = selffor _ in range(index):tmp = tmp.nexttmp.next = tmp.next.nextif index == self._length - 1:self._last = tmpself._length -= 1  # 更新链表长度def print_list(self):"""打印当前链表节点，用于测试，如下所示：index 1  2 3  ...value 11 2 33 ..."""tmp = self.nextidx = 0index_str = ''val_str = ''while tmp:interval = len(str(tmp.val)) - len(str(idx))index_str = index_str + str(idx) + ' ' * (1 if interval == 0 else 1 + interval if interval > 0 else 1)val_str = val_str + str(tmp.val) + ' ' * (1 if interval == 0 else 1 + abs(interval) if interval < 0 else 1)tmp = tmp.nextidx += 1print(index_str)print(val_str)def test_run(self, exc_str_list: list, val_list: list):"""测试，用于直接输入 LeetCode 测试数据进行测试说明：测试数据第一步为新建实例，需手动新建实例并删去对应值（如下所示），再调用"""for i in range(len(val_list)):print(exc_str_list[i], val_list[i])getattr(self, exc_str_list[i])(*val_list[i])self.print_list()if exc_str_list[i] == 'get':print(f'get({val_list[i][0]}) = ', getattr(self, exc_str_list[i])(*val_list[i]))print('length = ', self._length)print('==============================================')if __name__ == '__main__':MyLinkedList().test_run(["addAtHead", "addAtIndex", "addAtIndex", "addAtHead", "deleteAtIndex", "addAtIndex", "addAtHead","addAtTail", "addAtHead", "get", "addAtTail", "addAtTail", "addAtIndex", "addAtTail", "addAtTail","addAtHead", "addAtTail", "addAtHead", "addAtTail", "addAtHead", "addAtTail", "addAtTail", "addAtHead","addAtTail", "addAtIndex", "addAtHead", "deleteAtIndex", "addAtHead", "addAtHead", "addAtHead","addAtHead", "addAtIndex", "addAtHead", "addAtTail", "addAtHead", "deleteAtIndex", "addAtTail","deleteAtIndex", "addAtTail", "addAtTail", "addAtTail", "addAtTail", "addAtHead", "addAtTail","get", "addAtIndex", "get", "deleteAtIndex", "addAtTail", "addAtHead", "addAtTail", "addAtTail","addAtIndex", "addAtHead", "addAtHead", "addAtHead", "addAtHead", "addAtHead", "addAtTail", "deleteAtIndex","deleteAtIndex", "addAtHead", "addAtHead", "addAtTail", "addAtHead", "get", "addAtIndex", "addAtIndex", "get","addAtTail", "get", "addAtTail", "deleteAtIndex", "get", "addAtTail", "addAtTail", "addAtHead", "addAtTail","deleteAtIndex", "addAtHead", "addAtHead", "addAtHead", "deleteAtIndex", "addAtTail", "addAtIndex", "addAtTail","addAtTail", "addAtIndex", "addAtIndex", "addAtHead", "addAtIndex", "addAtHead", "addAtHead", "addAtTail","addAtIndex", "addAtTail", "get", "addAtHead", "addAtTail", "addAtHead", "addAtHead"],[[86], [1, 54], [1, 14], [83], [4], [3, 18], [46], [3], [76], [5], [79], [74], [7, 28], [68],[16], [82], [24], [30], [96], [21], [79], [66], [2], [2], [7, 57], [59], [21], [19], [55], [46],[17], [16, 41], [97], [85], [63], [30], [11], [16], [91], [29], [47], [20], [12], [80], [15], [12, 8],[21], [30], [11], [54], [51], [41], [8, 88], [62], [7], [59], [73], [73], [55], [9], [7], [49], [99],[17], [44], [11], [26, 86], [10, 99], [19], [71], [29], [32], [2], [3], [16], [2], [83], [31], [3], [23],[64], [96], [27], [81], [12, 78], [21], [69], [5, 35], [8, 72], [60], [19, 73], [55], [83], [86], [31, 70],[49], [19], [64], [22], [25], [13]])