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reorder_list.txt
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reorder_list.txt
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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 == NULL || head->next == NULL) return;
// head of first half
ListNode* l1 = head;
// head of second half
ListNode* slow = head;
// tail of second half
ListNode* fast = head;
// tail of first half
ListNode* prev = NULL;
while(fast != NULL && fast->next != NULL){
prev = slow;
slow = slow->next;
fast = fast->next->next;
}
prev->next = NULL;
ListNode* l2 = reverse(slow);
merge(l1, l2);
}
ListNode* reverse(ListNode* head){
ListNode* prev = NULL;
ListNode* curr = head;
while(curr != NULL){
ListNode* nxt = curr->next;
curr->next = prev;
prev = curr;
curr = nxt;
}
return prev;
}
void merge(ListNode* l1, ListNode* l2){
while(l1 != NULL){
ListNode* l1_next = l1->next;
ListNode* l2_next = l2->next;
l1->next = l2;
if(l1_next == NULL){
break;
}
l2->next = l1_next;
l1 = l1_next;
l2 = l2_next;
}
}
};