Leetcode 143. Reorder List (Medium) (cpp)

Leetcode 143. Reorder List (Medium) (cpp)

Tag: Linked List

Difficulty: Medium

/*

143. Reorder List (Medium)

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

*/
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode* head) {
        if (head == NULL || head -> next == NULL) {
            return;
        }
        ListNode *s = head, *f = head;
        while (f -> next != NULL && f -> next -> next != NULL) {
            s = s -> next;
            f = f -> next -> next;
        }
        ListNode *newhead = reverse(s -> next);
        s -> next = NULL;
        s = head;
        while (newhead != NULL) {
            ListNode *newheadpos = newhead -> next;
            newhead -> next = s -> next;
            s -> next = newhead;
            s = s -> next -> next;
            newhead = newheadpos;
        }
    }
private:
    ListNode* reverse(ListNode* head) {
        if (head == NULL || head -> next == NULL) {
            return head;
        }
        ListNode *newhead = reverse(head -> next);
        head -> next -> next = head;
        head -> next = NULL;
        return newhead;
    }
};