【leetcode】链表篇刷题 --

//删除指定value节点
class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        
        //单独处理head
        while(head != NULL && head->val == val){
            ListNode* temp = head;
            head = head->next;
            delete temp;
        }
        
        ListNode* current = head;
        //处理非head
        while(current != NULL && current->next != NULL){
            if(val == current->next->val){
                ListNode* temp = current->next;
                current->next = current->next->next;
                delete temp;
            }else{
                current = current->next;
            }
        }
        return head;
    }
};

/*
 * @lc app=leetcode.cn id=203 lang=cpp
 *
 * [203] 移除链表元素
 */

// @lc code=start
/**
 * 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 {
public:
    ListNode* removeElements(ListNode* head, int val) {
        //用于统一做删除处理
        ListNode* fHead = new ListNode(0);
        fHead->next = head;
        //用于遍历list
        ListNode* current = fHead;
        while(current->next != NULL){
            if(current->next->val == val){
                ListNode* temp = current->next;
                current->next = current->next->next;
                delete temp;
            }else{
                current = current->next;
            }
        }
        //返回时特别注意，经删除操作后，list已发生变化，返回虚拟头节点的next即可
        head = fHead->next;
        delete fHead;
        return head;
    }
};
// @lc code=end

/*
 * @lc app=leetcode.cn id=206 lang=cpp
 *
 * [206] 反转链表
 */

// @lc code=start
/**
 * 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 {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = nullptr;
        ListNode* curr = head;
        while(curr){
            ListNode* next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        //循环结束后curr=null
        return prev;

    }
};
// @lc code=end

/*
 * @lc app=leetcode.cn id=206 lang=cpp
 *
 * [206] 反转链表
 */

// @lc code=start
/**
 * 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 {
public:
//递归版的双指针法
    ListNode* reverseList(ListNode* head) {
        return reverse(nullptr, head);
       

    }
    ListNode* reverse(ListNode* prev, ListNode* cur){
        if(cur == nullptr){
            return prev;
        }
        ListNode* temp = cur->next;
        cur->next = prev;
		//注意这里cur，对应参数prev，temp对应参数cur
		//相当于将prev与cur分别下移一位
		//就等同于实现： prev = cur; cur = temp;
        return reverse(cur,temp);
    }
};
// @lc code=end

/*
 * @lc app=leetcode.cn id=24 lang=cpp
 *
 * [24] 两两交换链表中的节点
 */

// @lc code=start
/**
 * 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 {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode* dummyHand = new ListNode(0);
        dummyHand->next = head;
        ListNode* curr = dummyHand;
        //curr在完成第一对交换后，每次循环均指向上一次交换中的node1节点so 判断是否满足交换条件
        //需要含有 curr->next->next != nullptr
        //而curr->next!=nullptr是为了上面这句判断条件不报错（如果curr->next=null则curr->next->next != nullptr，空指针异常）
        while(curr->next != nullptr && curr->next->next != nullptr){
            ListNode* node1 = curr->next;
            ListNode* node2 = curr->next->next;
            curr->next = node2;
            node1->next = node2->next;
            node2->next = node1;
            curr = node1;
        }
        return dummyHand->next;
    }   
};
// @lc code=end