leetcode Reverse Linked List

最新推荐文章于 2020-03-28 23:36:09 发布

原创最新推荐文章于 2020-03-28 23:36:09 发布 · 414 阅读

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本文介绍了一种高效的方法来反转链表中指定区间的元素。通过一次遍历和常数空间复杂度实现，适用于解决LeetCode等平台上的相关问题。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *reverseBetween(ListNode *head, int m, int n) {
        if(m == n){
            return head;
        }
        ListNode dummy(-1);
        ListNode* prev = &dummy;
        dummy.next = head;
        ListNode* fast = prev, *slow = prev;
        for(int i = 1; i < m; ++i){ //fast points to the (m-1)-th pointer of the linked list
            fast = fast->next;
        }
        if(fast->next->next == NULL){
            return head;
        }
        ListNode *tail = fast; //tail: (m - 1)-th pointer of the linked list
        ListNode* tmphead = fast->next; //tmphead: m-th pointer of the linked list
        prev = fast; //(m - 1)-th pointer of the linked list
        fast = fast->next; //fast points to the m-th pointer of the linked list
        for(int i = m; i <= n; ++i){
            ListNode* tmpnext = fast->next; //(i + 1)-th pointer of the linked list
            fast->next = prev;
            prev = fast;
            fast = tmpnext;
        }
        //fast: (n + 1)-th pointer of the linked list
        tail->next = prev;
        tmphead->next = fast;
        return dummy.next;
    }
};

(7 ms solution, one pass and O(1) space)