代码随想录day4

24.两两交换链表：注意虚拟头节点的使用

ListNode* swapPairs(ListNode* head) {
        ListNode* dummy = new ListNode();
        dummy->next = head;
        ListNode* current = dummy;
        while(current->next != nullptr && current->next->next != nullptr){
            ListNode* tmp = current->next;
            ListNode* tmp1 = current->next->next->next;

            current->next = current->next->next;
            current->next->next = tmp;
            current->next->next->next = tmp1;
            current = current->next->next;
        }
        return dummy->next;
    }

19.删除链表中的倒数第N个结点：注意使用快慢指针，以及中间的间隔n的判断

ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* dummy = new ListNode();
        dummy->next = head;
        ListNode* fast = dummy;
        ListNode* slow = dummy;
        //先移动fast指针领先slow n+1 个位置
        while(n-- && fast != nullptr){
            fast = fast->next;
        }
        fast = fast->next;
        //同时移动fast，slow到fast抵达最后
        while(fast != nullptr){
            fast = fast->next;
            slow = slow->next;
        }
        //删除第N个需要用到的是第N-1个节点
        ListNode* tmp = slow->next;
        slow->next = tmp->next;
        delete(tmp);
        return dummy->next;
        
}

面试题02.07链表相交：//理解链表相交不是值相等，而是指向同一块区域

ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode* listA = headA;
        ListNode* listB = headB;
        int lenA = 0, lenB = 0;
        while(listA != nullptr){
            listA = listA->next;
            lenA++;
        }
        
        while(listB != nullptr){
            listB = listB->next;
            lenB++;
        }
        if(lenB > lenA){
            swap(headA, headB);
            swap(lenA, lenB);
        }
        int gap = lenA - lenB;
        listA = headA;
        listB = headB;
        while(gap--){
            listA = listA->next;
        }
        while(listA != nullptr && listB != nullptr){
            if(listA == listB){
                return listA;
            }else{
                listA = listA->next;
                listB = listB->next;
            }
        }
        return nullptr;
    }

142.环形链表II：//需理解为甚么fast比slow快1就能在环中相遇，什么时候fast追上slow，以及追上后如何获取环的入口位置

ListNode *detectCycle(ListNode *head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast != nullptr && fast->next != nullptr){
            fast = fast->next->next;
            slow = slow->next;
            //找到fast与slow在环中的相遇点
            if(fast == slow){
                ListNode* left = head;
                ListNode* right = fast;
                while(left != nullptr && right != nullptr){
                    if(left == right){
                        return left;
                    }
                    left = left->next;
                    right = right->next;

                }
                
            }
            
        }
        return nullptr;
    }