Reverse LinkedList - linked list implement

Question

This is simple question from the LeetCode : Reverse LinkedList I have two similar code and I can't figure it out what is difference between two but it outputs different result.

First while loop results correct answer but second one has wrong answer. Instead of temp = current. next, I just store current to temp and at the last line of the while loop, I correctly switch current to temp.next. I think they should result the same answer but when Input {1,2,3,4,5} then second solution got wrong answer {1}.

ListNode reverse = null;
ListNode current = head;

while(current != null){
  ListNode temp = current.next;
  current.next = reverse;
  reverse = current;
  current = temp;
}

Here is the second while loop.

while(current != null){
  ListNode temp = current;
  current.next = reverse;
  reverse = current;
  current = temp.next;
}

rahul_107 · Accepted Answer · 2019-06-05 05:02:51Z

1

while(current != null){
    // This line temp is keeping the reference  of current variable
    ListNode temp = current;
    // now temp and current both pointing to same value

    // This line will make the next of current variable pointing to reverse reference
    // as current and temp were same so next of temp is also pointing to the reverse
    current.next = reverse;

    // now the reverse will be changed and point to the current reference
    reverse = current;
    // so up to here reverse, temp, current all pointing to the same location
    // as all are pointing same so temp.next,curr.next and reverse.next
    // will store the previous reference 

    // current will point back to the reverse value
    // no increment is done in the whole loop
    current = temp.next;
}

answered Jun 5, 2019 at 5:02

rahul_107

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Comments

greybeard · Accepted Answer · 2019-06-05 04:55:08Z

0

In the second snippet, it may seem that current = temp.next sets current to its nextvalue from the top of the loop/the start or the current iteration - after all, nobody assigned something to temp.next.
But after temp = current, both are names for the same thing, and current.next = reverse overwrites something that will be needed and hasn't been saved.

answered Jun 5, 2019 at 4:55

greybeard

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