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  • [LeetCode] 876. Middle of the Linked List

    Description

    Given a non-empty, singly linked list with head node head, return a middle node of linked list.

    If there are two middle nodes, return the second middle node.

    Example 1:

    Input: [1,2,3,4,5]
    Output: Node 3 from this list (Serialization: [3,4,5])
    The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
    Note that we returned a ListNode object ans, such that:
    ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
    

    Example 2:

    Input: [1,2,3,4,5,6]
    Output: Node 4 from this list (Serialization: [4,5,6])
    Since the list has two middle nodes with values 3 and 4, we return the second one.
    

    Note:

    • The number of nodes in the given list will be between 1 and 100.

    Analyse

    找单链表中点,一看到题就想到快慢指针

    我的思路是开始fast, slow两个指针都在head处,fast每次走两步,slow每次走一步,再考虑到链表长度的奇偶可能导致fast走不了两步,加了个判断

    ListNode* middleNode(ListNode* head) {
        if (head == NULL)
        {
            return head;
        }
        
        ListNode* fast = head;
        ListNode* slow = head;
        
        while (fast->next && fast->next->next)
        {
            fast = fast->next->next;
            slow = slow->next;
        }
        
        if (fast->next)
        {
            slow = slow->next;
        }
        
        return slow;
    }
    

    中间遇到下面这个问题,加上了几个NULL的判断就通过编译了

    member access within null pointer of type 'struct ListNode'

    再看看leetcode上其他人写的快慢指针,同样的快慢指针,这个明显要更好,不用考虑链表长度

    ListNode* middleNode(ListNode* head) {
        if(head==NULL || head->next==NULL) 
            return head;
        ListNode* slow=head;
        ListNode* fast=head;
        fast=fast->next->next;
        while(fast!=NULL && fast->next!=NULL){
            slow=slow->next;
            fast=fast->next->next;
        }
        return slow->next;
    }
    

    References

    1. LeetCodeBug-member access within null pointer of type 'struct ListNode'
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  • 原文地址:https://www.cnblogs.com/arcsinw/p/9397682.html
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