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// 143. Reorder List
#include "leetcode.h"
/*
* given the head of a singly linked list. the list can be represented as 'l0 ->
* l1 -> ... -> Ln'. reorder the list to be of the following form: 'l0 -> ln ->
* ln-1 -> ln-2'. you may not modify the values in the list's nodes. only nodes
* themselves may be changed.
*/
struct ListNode {
int val;
struct ListNode *next;
};
struct ListNode *get_mid(struct ListNode *head) {
struct ListNode *fast = head, *slow = head;
while (fast->next && fast->next->next) {
fast = fast->next->next;
slow = slow->next;
}
return slow;
}
struct ListNode *reverse(struct ListNode *head) {
struct ListNode *tail = NULL;
while (head) {
struct ListNode *curr = head;
head = head->next;
curr->next = tail;
tail = curr;
}
return tail;
}
void merge(struct ListNode *n, struct ListNode *m) {
struct ListNode *head = n;
while (m) {
struct ListNode *tmp = m->next;
m->next = head->next;
head->next = m;
head = head->next->next;
m = tmp;
}
}
void reorderList(struct ListNode *head) {
if (!head || !head->next || !head->next->next)
return;
struct ListNode *mid = get_mid(head), *half = mid->next;
mid->next = NULL;
half = reverse(half);
merge(head, half);
}
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