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‎Problemset/0025-Reverse Nodes in k-Group/README.md

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# 25. Reverse Nodes in k-Group
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Given the `head` of a linked list, reverse the nodes of the list `k` at a time, and return *the modified list*.
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`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.
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You may not alter the values in the list's nodes, only nodes themselves may be changed.
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#### Example 1:
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![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex1.jpg)
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<pre>
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<strong>Input:</strong> head = [1,2,3,4,5], k = 2
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<strong>Output:</strong> [2,1,4,3,5]
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</pre>
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#### Example 2:
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![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex2.jpg)
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<pre>
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<strong>Input:</strong> head = [1,2,3,4,5], k = 3
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<strong>Output:</strong> [3,2,1,4,5]
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</pre>
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#### Constraints:
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* The number of nodes in the list is `n`.
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* `1 <= k <= n <= 5000`
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* `0 <= Node.val <= 1000`
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**Follow-up:** Can you solve the problem in `O(1)` extra memory space?
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## Solutions (Python)
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### 1. Solution
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```Python
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# Definition for singly-linked list.
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# class ListNode:
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# def __init__(self, val=0, next=None):
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# self.val = val
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# self.next = next
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class Solution:
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def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
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length = 0
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curr = head
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while curr is not None:
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length += 1
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curr = curr.next
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dummy = ListNode(next=head)
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grouptail = dummy
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for _ in range(length // k):
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grouphead = grouptail.next
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prev = grouphead
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for _ in range(k):
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prev = prev.next
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curr = grouphead
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for _ in range(k):
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temp = curr
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curr = curr.next
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temp.next = prev
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prev = temp
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grouptail.next = prev
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grouptail = grouphead
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return dummy.next
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```

‎Problemset/0025-Reverse Nodes in k-Group/README_CN.md

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# 25. K 个一组翻转链表
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给你链表的头节点 `head` ，每 `k` 个节点一组进行翻转，请你返回修改后的链表。
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`k` 是一个正整数，它的值小于或等于链表的长度。如果节点总数不是 `k` 的整数倍，那么请将最后剩余的节点保持原有顺序。
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你不能只是单纯的改变节点内部的值，而是需要实际进行节点交换。
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#### 示例 1:
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![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex1.jpg)
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<pre>
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<strong>输入:</strong> head = [1,2,3,4,5], k = 2
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<strong>输出:</strong> [2,1,4,3,5]
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</pre>
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#### 示例 2:
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![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex2.jpg)
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<pre>
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<strong>输入:</strong> head = [1,2,3,4,5], k = 3
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<strong>输出:</strong> [3,2,1,4,5]
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</pre>
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#### 提示:
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* 链表中的节点数目为 `n`
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* `1 <= k <= n <= 5000`
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* `0 <= Node.val <= 1000`
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**进阶：**你可以设计一个只用 `O(1)` 额外内存空间的算法解决此问题吗？
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## 题解 (Python)
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### 1. 题解
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```Python
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# Definition for singly-linked list.
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# class ListNode:
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# def __init__(self, val=0, next=None):
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# self.val = val
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# self.next = next
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class Solution:
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def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
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length = 0
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curr = head
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while curr is not None:
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length += 1
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curr = curr.next
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dummy = ListNode(next=head)
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grouptail = dummy
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for _ in range(length // k):
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grouphead = grouptail.next
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prev = grouphead
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for _ in range(k):
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prev = prev.next
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curr = grouphead
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for _ in range(k):
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temp = curr
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curr = curr.next
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temp.next = prev
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prev = temp
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grouptail.next = prev
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grouptail = grouphead
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return dummy.next
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```

‎Problemset/0025-Reverse Nodes in k-Group/Solution.py

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# Definition for singly-linked list.
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# class ListNode:
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# def __init__(self, val=0, next=None):
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# self.val = val
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# self.next = next
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class Solution:
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def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
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length = 0
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curr = head
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while curr is not None:
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length += 1
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curr = curr.next
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dummy = ListNode(next=head)
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grouptail = dummy
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for _ in range(length // k):
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grouphead = grouptail.next
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prev = grouphead
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for _ in range(k):
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prev = prev.next
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curr = grouphead
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for _ in range(k):
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temp = curr
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curr = curr.next
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temp.next = prev
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prev = temp
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grouptail.next = prev
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grouptail = grouphead
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return dummy.next

‎README.md

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[21][21l] |[Merge Two Sorted Lists][21] |![py]
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[22][22l] |[Generate Parentheses][22] |![rs]
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[24][24l] |[Swap Nodes in Pairs][24] |![py]
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[25][25l] |[Reverse Nodes in k-Group][25] |![py]
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[26][26l] |[Remove Duplicates from Sorted Array][26] |![rs]
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[27][27l] |[Remove Element][27] |![rb]&nbsp;&nbsp;![rs]
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[28][28l] |[Implement strStr()][28] |![py]
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[21]:Problemset/0021-Merge%20Two%20Sorted%20Lists/README.md#21-merge-two-sorted-lists
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[22]:Problemset/0022-Generate%20Parentheses/README.md#22-generate-parentheses
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[24]:Problemset/0024-Swap%20Nodes%20in%20Pairs/README.md#24-swap-nodes-in-pairs
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[25]:Problemset/0025-Reverse%20Nodes%20in%20k-Group/README.md#25-reverse-nodes-in-k-group
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[26]:Problemset/0026-Remove%20Duplicates%20from%20Sorted%20Array/README.md#26-remove-duplicates-from-sorted-array
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[27]:Problemset/0027-Remove%20Element/README.md#27-remove-element
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[28]:Problemset/0028-Implement%20strStr\(\)/README.md#28-implement-strstr
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[21l]:https://leetcode.com/problems/merge-two-sorted-lists/
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[22l]:https://leetcode.com/problems/generate-parentheses/
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[24l]:https://leetcode.com/problems/swap-nodes-in-pairs/
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[25l]:https://leetcode.com/problems/reverse-nodes-in-k-group/
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[26l]:https://leetcode.com/problems/remove-duplicates-from-sorted-array/
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[27l]:https://leetcode.com/problems/remove-element/
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[28l]:https://leetcode.com/problems/implement-strstr/

‎README_CN.md

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[21][21l] |[合并两个有序链表][21] |![py]
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[22][22l] |[括号生成][22] |![rs]
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[24][24l] |[两两交换链表中的节点][24] |![py]
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[25][25l] |[K 个一组翻转链表][25] |![py]
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[26][26l] |[删除排序数组中的重复项][26] |![rs]
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[27][27l] |[移除元素][27] |![rb]&nbsp;&nbsp;![rs]
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[28][28l] |[实现 strStr()][28] |![py]
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[21]:Problemset/0021-Merge%20Two%20Sorted%20Lists/README_CN.md#21-合并两个有序链表
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[22]:Problemset/0022-Generate%20Parentheses/README_CN.md#22-括号生成
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[24]:Problemset/0024-Swap%20Nodes%20in%20Pairs/README_CN.md#24-两两交换链表中的节点
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[25]:Problemset/0025-Reverse%20Nodes%20in%20k-Group/README_CN.md#25-k-个一组翻转链表
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[26]:Problemset/0026-Remove%20Duplicates%20from%20Sorted%20Array/README_CN.md#26-删除排序数组中的重复项
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[27]:Problemset/0027-Remove%20Element/README_CN.md#27-移除元素
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[28]:Problemset/0028-Implement%20strStr\(\)/README_CN.md#28-实现-strstr
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[21l]:https://leetcode.cn/problems/merge-two-sorted-lists/
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[22l]:https://leetcode.cn/problems/generate-parentheses/
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[24l]:https://leetcode.cn/problems/swap-nodes-in-pairs/
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[25l]:https://leetcode.cn/problems/reverse-nodes-in-k-group/
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[26l]:https://leetcode.cn/problems/remove-duplicates-from-sorted-array/
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[27l]:https://leetcode.cn/problems/remove-element/
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[28l]:https://leetcode.cn/problems/implement-strstr/

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