206. Reverse Linked List

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本文介绍了一种将单链表逆序的方法，包括递归和迭代两种实现方式。递归方法通过反转指针来实现链表逆序；迭代方法则通过三个指针前、中、后来逐个节点地修改节点指向，最终达到逆序效果。

Reverse a singly linked list.

recursively

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode* p = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return p;
    }
};

iteratively

用了vector存下来。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode* p = head;
        ListNode* q = new ListNode(0);
        q->next = p;
        vector<int> v;
        while(head){
            v.push_back(head->val);
            head = head->next;
        }
        for(int i = v.size()-1; i>=0;--i){
            p->val = v[i];
            p = p->next;
        }
        return q->next;
    }
};

iteratively 直接改

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        
        ListNode* pre = NULL;
        while(head){
            ListNode* next = head->next;   //先保存下来 不然下一条语句会改变
            head->next = pre;
            pre = head;
            head = next;
        }
        return pre;
        
    }
};