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143.cpp
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143.cpp
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/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode() : val(0), next(nullptr) {}
* ListNode(int x) : val(x), next(nullptr) {}
* ListNode(int x, ListNode *next) : val(x), next(next) {}
* };
*/
class Solution {
public:
void reorderList(ListNode* head) {
if (head == nullptr) return;
// step 1: get the middle
// OOOOO OOOOOO
// ^ ^
ListNode* slow, *fast;
slow = fast = head;
while (fast && fast->next) {
slow = slow->next;
fast = fast->next->next;
}
// step 2: reverse
ListNode* prev = nullptr, *curr = slow, *temp;
while (curr) {
temp = curr->next;
curr->next = prev;
prev = curr;
curr = temp;
}
// step 3: merge
ListNode* firstHead = head;
ListNode* sceondHead = prev;
while (sceondHead->next) {
temp = firstHead->next;
firstHead->next = sceondHead;
firstHead = temp;
temp = sceondHead->next;
sceondHead->next = firstHead;
sceondHead = temp;
}
}
};