143. Reorder List

O(n) time O(1) space

/**
 * 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) {
        ListNode dummy_head(0);
        dummy_head.next = head;
        auto slow = &dummy_head, fast = slow; // 如果希望最后slow落在(n - 1) / 2的那个，要从dummy_head开始
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        fast = slow->next;
        slow->next = nullptr;
        ListNode *prev = nullptr, *curr = fast;
        while (curr) {
            auto next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        fast = prev;
        slow = head;
        while (fast) {
            auto next = fast->next;
            fast->next = slow->next;
            slow->next = fast;
            slow = fast->next;
            fast = next;
        }
    }
};

O(n) time O(n) space

/**
 * 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) {
        ListNode dummy_head(0);
        dummy_head.next = head;
        auto slow = &dummy_head, fast = slow;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        fast = slow->next;
        slow->next = nullptr;
        stack<ListNode *> s;
        while (fast) {
            s.push(fast);
            fast = fast->next;
        }
        slow = head;
        while (!s.empty()) {
            auto next = slow->next;
            s.top()->next = next;
            slow->next = s.top(); s.pop();
            slow = next;
        }
    }
};