143. Reorder List

143. Reorder List

題意

給定一個 linked list

將 $L_0$→$L_1$ → … →$L_{n - 1}$→$L_n$ 改為 $L_0$→$L_n$→$L_1$→$L_{n-1}$→$L_2$→$L_{n-2}$ → …

限制

• The number of nodes in the list is in the range [1, 5 * 10^4].

• 1 <= Node.val <= 1000

思考

• 額外用陣列儲存每個節點後按 index 重新排列，可用 O(n) Space complexity 完成

• 若要 inplace 的完成 reorder

  • 找出中心點，分為兩半 ⇒ 快慢指標

  • 將右半邊的 linked list 反過來

  • 合併兩條 linked list

Solution

/**
 * 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 {
    ListNode* reverse(ListNode* head) {
        auto node = head;
        ListNode* prev = nullptr;
        while (node) {
            auto next = node->next;
            node->next = prev;
            prev = node;
            node = next;
        }
        return prev;
    }
public:
    void reorderList(ListNode* head) {
        auto fast = head;
        auto slow = head;
        while (fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
        }

        auto mid = reverse(slow);

        while (mid && mid->next) {
            auto nextHead = head->next;
            auto nextMid = mid->next;
            head->next = mid;
            mid->next = nextHead;
            head = nextHead;
            mid = nextMid;
        }
    }
};