day2

1.剑指 Offer 06. 从尾到头打印链表

 1)常规法，遍历+反向迭代器

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     vector<int> reversePrint(ListNode* head) {
12       vector<int> res;
13       while(head){
14           res.push_back(head -> val);
15           head = head -> next;
16       }
17       return vector<int>(res.rbegin(),res.rend());
18     }
19 };

 2)递归法，遍历到最后刚好 从最后一个数据开始往容器里放数据

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     vector<int> res;
12 
13     vector<int> reversePrint(ListNode* head) {
14       recur(head);
15       return res;
16     }
17     
18     void recur(ListNode* head){
19         if(head == nullptr)
20          return;
21         recur(head -> next);
22         res.push_back(head -> val);
23     }
24 };

2.剑指 Offer 24. 反转链表

 1)

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* reverseList(ListNode* head) {
12       ListNode* p = nullptr;
13       while(head){
14           ListNode* tmp = head -> next;
15           head -> next = p;
16           p = head;
17           head = tmp;
18       } 
19       return p;
20     }
21 };

 2)递归（ 头节点那里是真的妙！）

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* reverseList(ListNode* head) {
12       return recur(nullptr,head);//为什么不从head开始:方便处理空链表和只有一个数据的链表
13     }
14 
15     ListNode* recur(ListNode* pre,ListNode* cur){
16       if(cur == nullptr) return pre;
17       ListNode* res = recur(cur,cur -> next);
18       cur -> next = pre;//反串
19       return res;
20     }//res的作用仅仅就是保存头节点!!!
21 };

 3.剑指 Offer 35. 复杂链表的复制

 1)哈希表，key:节点，value: 节点

 1 /*
 2 // Definition for a Node.
 3 class Node {
 4 public:
 5     int val;
 6     Node* next;
 7     Node* random;
 8     
 9     Node(int _val) {
10         val = _val;
11         next = NULL;
12         random = NULL;
13     }
14 };
15 */
16 class Solution {
17 public:
18     Node* copyRandomList(Node* head) {
19        if(head == nullptr)
20         return nullptr;
21        unordered_map<Node*,Node*> map;
22        Node* cur = head;
23        while(cur){
24            map[cur] = new Node(cur -> val);
25            cur = cur -> next;
26        }
27        cur = head;
28        while(cur){
29            map[cur] -> next = map[cur -> next];
30            map[cur] -> random = map[cur -> random];
31            cur = cur -> next;
32        }
33        return map[head];
34     }
35 };

 2)复制拼接+拆分 

 1 /*
 2 // Definition for a Node.
 3 class Node {
 4 public:
 5     int val;
 6     Node* next;
 7     Node* random;
 8     
 9     Node(int _val) {
10         val = _val;
11         next = NULL;
12         random = NULL;
13     }
14 };
15 */
16 class Solution {
17 public:
18     Node* copyRandomList(Node* head) {
19        if(head == nullptr)
20         return nullptr;
21        Node* cur = head;
22        while(cur){
23            Node* tmp = new Node(cur -> val);
24            tmp -> next = cur -> next;
25            cur -> next = tmp;
26            cur = tmp -> next;
27        }
28        cur = head;
29        while(cur){
30          if(cur -> random)
31            cur -> next -> random = cur -> random -> next;//创建的新节点next和random都是nullptr，所以要设置 //I
32          cur = cur -> next -> next;
33        }
34        cur = head -> next;
35        Node* pre = head,*res = head -> next;//res保存头节点
36        while(cur -> next){
37            pre -> next = pre -> next -> next;
38            cur -> next = cur -> next -> next;
39            pre = pre -> next;
40            cur = cur -> next;
41        }
42        pre -> next = nullptr;//处理原链表尾节点
43        return res;
44     }
45 };
46 /*
47        cur = head -> next;   //II
48        Node* pre = head,*res = head -> next;//res保存头节点
49        while(cur -> next){
50            cur -> random = pre -> random -> next;//创建的新节点next和random都是nullptr，所以要设置
51            pre -> next = pre -> next -> next;
52            cur -> next = cur -> next -> next;
53            pre = pre -> next;
54            cur = cur -> next;
55        }
56 */

I.注意第31行:cur->next->random = cur -> random 是错误的，cur ->next->random保存的random值应该是cur->random保存的值即 cur->random->next

II.注意47-55行，我之前是想把28-41行改为47-55行，发现不行，因为最后一个(cur)节点会分配不到random值

posted @ 2022-06-26 23:01  balabalahhh  阅读(43)  评论(0)    收藏  举报