【LC刷题】DAY02：24 19 142

【LC刷题】DAY02：24 19 142

24. 两两交换链表中的节点 link

/**
 * 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* dummyHead  = new ListNode(0);
        dummyHead->next = head;
        ListNode* temp = dummyHead;
        while(temp->next!=nullptr && temp->next->next != nullptr){
            ListNode *node1 = temp->next;
            ListNode *node2 = node1->next;
            temp->next = node2;
            node1->next = node2->next;
            node2 -> next = node1;
            temp = node1;
        }

        return dummyHead->next;
    }
};

19.删除链表的倒数第N个节点 link

class Solution {
public:
    int getLength(ListNode* head) {
        int length = 0;
        while (head) {
            ++length;
            head = head->next;
        }
        return length;
    }

    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* dummy = new ListNode(0, head);
        int length = getLength(head);
        ListNode* cur = dummy;
        for (int i = 1; i < length - n + 1; ++i) {
            cur = cur->next;
        }
        cur->next = cur->next->next;
        ListNode* ans = dummy->next;
        delete dummy;
        return ans;
    }
};

面试题 02.07. 链表相交 link

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        unordered_set<ListNode *> visited;
        ListNode *temp = headA;
        while (temp != nullptr) {
            visited.insert(temp);
            temp = temp->next;
        }
        temp = headB;
        while (temp != nullptr) {
            if (visited.count(temp)) {
                return temp;
            }
            temp = temp->next;
        }
        return nullptr;
    }
};

142.环形链表II

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

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