算法训练第三天

203.移除链表元素

思路：

很经典的链表算法。

代码：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):def removeElements(self, head, val):""":type head: Optional[ListNode]:type val: int:rtype: Optional[ListNode]"""if head is None:return headnow = ListNode(next=head)pos = nowwhile pos.next is not None:if pos.next.val == val:pos.next = pos.next.nextelse:pos = pos.nextreturn now.next

707.设计链表

思路：

这里题目说可以选择使用单链表或者双链表，设计并实现自己的链表。那我们就创建一个Node类去实现自己的链表，这里我选用的是单链表。

代码：

class Node:def __init__(self, val=0, next=None):self.val = valself.next = nextclass MyLinkedList(object):def __init__(self):self.head = Node()def get(self, index):""":type index: int:rtype: int"""pos = self.headi = 0while i <= index and pos is not None:pos = pos.nexti+=1if pos:return pos.valelse:return -1def addAtHead(self, val):""":type val: int:rtype: None"""new_node = Node(val=val, next=self.head.next)self.head.next = new_nodedef addAtTail(self, val):""":type val: int:rtype: None"""new_node = Node(val=val)pos = self.headwhile pos.next is not None:pos = pos.nextpos.next = new_nodedef addAtIndex(self, index, val):""":type index: int:type val: int:rtype: None"""new_node = Node(val=val)pos = self.headi = 0while i < index and pos.next is not None:pos = pos.nexti+=1if i==index:new_node.next = pos.nextpos.next = new_nodedef deleteAtIndex(self, index):""":type index: int:rtype: None"""pos = self.headi = 0while i < index and pos.next is not None:pos = pos.nexti+=1if pos.next:pos.next = pos.next.next

206.反转链表

思路：

我们可以新建一个头节点，指向为空，然后可以将给定链表使用头插入的方式逐个插入新的链表。

代码：

class Solution(object):def reverseList(self, head):""":type head: ListNode:rtype: ListNode"""new_head = ListNode()pos = headwhile pos is not None:tmp = pospos = pos.nexttmp.next=new_head.nextnew_head.next= tmpreturn new_head.next