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25.cpp
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70 lines (65 loc) · 1.75 KB
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// Author: btjanaka (Bryon Tjanaka)
// Problem: (LeetCode) 25
// Title: Reverse Nodes in k-Group
// Link: https://leetcode.com/problems/reverse-nodes-in-k-group/
// Idea: Mostly implement the problem; pointers get really tricky when reversing
// the list.
// Difficulty: medium
// Tags: linked-list, implementation
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseKGroup(ListNode* head, int k) {
if (head == NULL) return head;
ListNode* tail = new ListNode(0);
tail->next = head;
head = tail;
int i = 0;
for (ListNode* cur = tail->next; cur != NULL; cur = cur->next) {
if (i % k == 0 && i != 0) {
ListNode* reverse_itr = tail->next;
ListNode* next = reverse_itr->next;
for (;;) {
if (next == cur) {
tail->next->next = cur;
ListNode* tmp = tail->next;
tail->next = reverse_itr;
tail = tmp;
break;
}
ListNode* nextnext = next->next;
next->next = reverse_itr;
reverse_itr = next;
next = nextnext;
}
}
++i;
}
// edge case at the end - do it one more time :p
if (i % k == 0) {
ListNode* reverse_itr = tail->next;
ListNode* next = reverse_itr->next;
for (;;) {
if (next == NULL) {
tail->next->next = NULL;
ListNode* tmp = tail->next;
tail->next = reverse_itr;
tail = tmp;
break;
}
ListNode* nextnext = next->next;
next->next = reverse_itr;
reverse_itr = next;
next = nextnext;
}
}
return head->next;
}
};