Reverse Linked List

# Definition for singly-linked list. # class ListNode(object): # def __init__(self, x): # self.val = x # self.next = None class Solution(object): def reverse(self, head): """ input: ListNode head return: ListNode """ # write your solution here if head == None: return head prev = None curr = head while (curr != None and curr.next != None): ### We break out the loop when we have current as the last element. All elements till then has the same operation. The last element needs extra care so we deal with it outside the loop. ### action in every round of loop # keep control of the next node after = curr.next # swap curr to point to prev curr.next = prev # update curr, prev and next so we proceed to the next group of node prev = curr curr = after after = after.next # break out the loop. Now we have curr at the last node! But it has not been reversed yet. curr.next = prev head = curr return head