|
| 1 | +""" |
| 2 | +Use a dummy head, and |
| 3 | + |
| 4 | +l, r : define reversing range |
| 5 | + |
| 6 | +pre, cur : used in reversing, standard reverse linked linked list method |
| 7 | + |
| 8 | +jump : used to connect last node in previous k-group to first node in following k-group |
| 9 | +""" |
| 10 | +class Solution: |
| 11 | + def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]: |
| 12 | + # Dummy node initialization |
| 13 | + dummy = jump = ListNode(-1) |
| 14 | + dummy.next = l = r = head |
| 15 | + |
| 16 | + while True: |
| 17 | + count = 0 |
| 18 | + # Moving k positions |
| 19 | + while r and count < k: |
| 20 | + count += 1 |
| 21 | + r = r.next |
| 22 | + # If count == k, then reverse the segment [l, r] |
| 23 | + if count == k: |
| 24 | + pre, cur = r, l |
| 25 | + for _ in range(k): |
| 26 | + temp = cur.next |
| 27 | + cur.next = pre |
| 28 | + pre = cur |
| 29 | + cur = temp |
| 30 | + # Joining the current k-group with the previous k-group |
| 31 | + jump.next = pre |
| 32 | + jump = l |
| 33 | + l = r |
| 34 | + else: |
| 35 | + return dummy.next |
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