2017-10-29 @sunderls

https://leetcode.com/problems/reverse-nodes-in-k-group/description/

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

You may not alter the values in the nodes, only nodes itself may be changed.

Only constant memory is allowed.

For example, Given this linked list:

`1->2->3->4->5`

For k = 2, you should return:

`2->1->4->3->5`

For k = 3, you should return:

`3->2->1->4->5`

we've done a special case(k = 2) in our last article leetcode war 24 - swap nodes in pairs, it cost us O(n) with three cursors.

for a general k, we can still accomplish it with a constant 3 cursor, like:

```
head -> 1 -> 2 -> 3 -> 4
head -> 3 -> 1 -> 2 -> 4
head -> 3 -> 2 -> 1 -> 4
```

but after iteration 2, we have to move p3(3) to 2, this needs a for loop, this accumulates to O(k^{2})

we can try another way

```
head -> 1 -> 2 -> 3 -> 4
head -> 2 -> 1 -> 3 -> 4
head -> 3 -> 2 -> 1 -> 4
```

which means we move p2 along the way, and swap p2.next with p1.next. and when p2 went after k - 1 round, update p1 to p2.

but *the left-out nodes should remain as it is* so we have to know where kth node is before setting p2 to the next

```
var reverseKGroup = function(head, k) {
// fake head
var start = new ListNode();
start.next = head;
// use some pointers
var p1 = start;
// pk is used to run ahead to determine whether start swapping
var pk = p2 = p1.next;
var p3 = null;
var pknext = null;
// a counter
var count = k - 1;
while (pk) {
// first find the k-ahead node
pknext = pk;
count = k - 1;
while (count > 0 && pknext) {
pknext = pknext.next;
count -= 1;
}
// if there is this node
// safely do swapping for the next k - 1 round
if (pknext) {
count = k - 1;
while (count > 0) {
// swap p2.next with p1.next
p3 = p2.next;
p2.next = p3.next;
p3.next = p1.next;
p1.next = p3;
count -= 1;
}
// after swaping reset all to next start line
p1 = p2;
pk = p2 = p2.next
} else {
break;
}
}
return start.next;
};
```

It passed, ranking the top, fastest 119ms, wow!