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;}