Q2.6 Find the Beginning of the loop

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Q:Given a circular linked list, implement an algorithm which returnsnode at the beginning of the loop.

思路1：可以用哈希表记录每一个节点，遍历链表，通过哈希表记录判断，如果当前节点已经被访问过，那么有环。如果遍历结束，那么没有loop

思路2：

1、双指针查看是否有环。快指针速度为2，慢指针速度为1

2、如果有环,则再利用双指针p = head, q = 快慢指针交点; p, q速度都是1，相等的时候就是loop的起点

#include <iostream>using namespace std;struct ListNode {    int val;ListNode *next; ListNode(int x) : val(x), next(NULL) {} }; ListNode *init(int a[], int n) { ListNode *head = NULL; ListNode *p = NULL; for (int i = 0; i < n; i++) { ListNode *cur = new ListNode(a[i]); if (i == 0) { head = cur; p = cur; } p->next = cur; p = cur; } return head;}ListNode *detectCycle(ListNode *head) {        if (!head) {            return NULL;        }        ListNode *slow = head;        ListNode *fast = head;        while (fast && fast->next) {            slow = slow->next;            fast = fast->next->next;            if (slow == fast) {                ListNode *slow2 = head;                while (slow2 != slow) {                    slow = slow->next;                    slow2 = slow2->next;                }                return slow2;            }        }        return NULL;}void printList(ListNode *head) {ListNode *p = head;for( ; p; p = p->next) {cout<<p->val<<" ";}cout<<endl;}int main() {int a[10] = {2,4,3};int b[10] = {5,6,4};ListNode *head1 = init(a, 3);ListNode *head2 = init(b, 3);printList(head1);printList(head2);ListNode *head = addTwoNumbers(head1, head2);printList(head);return 0;}

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