LeetCode 143. Reorder List

Posted 2020-10-09

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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 || !head->next || !head->next->next) return;
        ListNode *fast = head;
        ListNode *slow = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;        
        }
        ListNode *mid = slow->next;
        slow->next = NULL;
        ListNode *curr = mid;
        ListNode *prev = NULL;
        
        while(curr) {
            ListNode *tmp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = tmp;
        }
        
        while(head && prev) {
            ListNode *next = head->next;
            head->next = prev;
            prev = prev->next;
            head->next->next = next;
            head = next;
        }        
    }    
};