算法二刷day4

24. 两两交换链表中的节点

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode *dummyHead = new ListNode(0);
        dummyHead->next = head;
        ListNode *cur = dummyHead;
        while (cur->next != nullptr && cur->next->next != nullptr) {
            ListNode *tmp = cur->next;
            cur->next = cur->next->next;
            tmp->next = cur->next->next;
            cur->next->next = tmp;
            cur = cur->next->next;
        }
        return dummyHead->next;
    }
};

19.删除链表的倒数第N个节点

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode *dummyHead = new ListNode(0);
        dummyHead->next = head;
        ListNode *cur = dummyHead;
        int count = 0;
        while (cur != nullptr) {
            count++;
            cur = cur->next;
        }
        // if (n > count) return nullptr;
        int k = count - n - 1;
        cur = dummyHead;
        while (k--) {
            cur = cur->next;
        }
        ListNode *tmp = cur->next;
        cur->next = cur->next->next;
        delete tmp;
        tmp = nullptr;
        return dummyHead->next;
    }
};

面试题 02.07. 链表相交

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode *curA = headA;
        ListNode *curB = headB;
        int countA = 0, countB = 0;
        while (curA != NULL) {
            countA++;
            curA = curA->next;
        }
        while (curB != NULL) {
            countB++;
            curB = curB->next;
        }
        int k;
        if (countA > countB) {
            curA = headB;
            curB = headA;
            k = countA - countB;
        }else {
            curA = headA;
            curB = headB;
            k = countB - countA;
        }
        while (k--) {
            curB = curB->next;
        }
        while (curB != NULL) {
            if (curA == curB) return curA;
            else {
                curA = curA->next;
                curB = curB->next;
            }
        }
        return NULL;
    }
};

142.环形链表II

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *cur1 = head;
        ListNode *cur2 = head;
        while (cur2 != NULL && cur2->next != NULL) {
            cur1 = cur1->next;
            cur2 = cur2->next->next;
            if (cur1 == cur2) {
                ListNode *index1 = head;
                ListNode *index2 = cur1;
                while (index1 != index2) {
                    index1 = index1->next;
                    index2 = index2->next;
                }
                return index1;
            }
        }
        return NULL;
    }
};