linear-list/linked-list/reverse-linked-list-ii
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2023-12-01
Reverse Linked List II
描述
Reverse a linked list from position m
to n
. Do it in-place and in one-pass.
For example: Given 1->2->3->4->5->nullptr
, m
= 2 and n
= 4,
return 1->4->3->2->5->nullptr
.
Note: Given m
, n
satisfy the following condition:
$$1 \leq m \leq n \leq $$ length of list.
分析
这题非常繁琐,有很多边界检查,15 分钟内做到 bug free 很有难度!
代码
// Reverse Linked List II
// 迭代版,时间复杂度O(n),空间复杂度O(1)
public class Solution {
public ListNode reverseBetween(ListNode head, int m, int n) {
ListNode dummy = new ListNode(-1);
dummy.next = head;
ListNode prev = dummy;
for (int i = 0; i < m-1; ++i)
prev = prev.next;
ListNode head2 = prev;
prev = head2.next;
ListNode cur = prev.next;
for (int i = m; i < n; ++i) {
prev.next = cur.next;
cur.next = head2.next;
head2.next = cur; // 头插法
cur = prev.next;
}
return dummy.next;
}
};
// Reverse Linked List II
// 迭代版,时间复杂度O(n),空间复杂度O(1)
class Solution {
public:
ListNode *reverseBetween(ListNode *head, int m, int n) {
ListNode dummy(-1);
dummy.next = head;
ListNode *prev = &dummy;
for (int i = 0; i < m-1; ++i)
prev = prev->next;
ListNode* const head2 = prev;
prev = head2->next;
ListNode *cur = prev->next;
for (int i = m; i < n; ++i) {
prev->next = cur->next;
cur->next = head2->next;
head2->next = cur; // 头插法
cur = prev->next;
}
return dummy.next;
}
};