【leetcode】【单链表】【142】Linked List Cycle II_leetcode142 linked list cycle ii.using namespace s-优快云博客

本文链接：https://blog.youkuaiyun.com/ruan875417/article/details/45645295

本文介绍了一种使用快慢指针检测链表中循环的有效方法，并提供了完整的C++实现代码。该方法首先通过快慢指针判断链表是否存在循环，若存在则进一步定位循环起点。

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#include<iostream>
using namespace std;

struct ListNode {
	int val;
	ListNode *next;
	ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
	ListNode *detectCycle(ListNode *head) {
		ListNode* slow = head;
		ListNode* fast = head;
		while (fast&&fast->next){
			slow = slow->next;
			fast = fast->next->next;
			if (slow == fast){
				ListNode* cur = head;
				while (cur != slow){
					cur = cur->next;
					slow = slow->next;
				}
				return cur;
			}
		}
		return NULL;
	}
	ListNode* createList(ListNode* head){
		int numOfNode;
		int value;
		cout << "please input number of listNode:";
		cin >> numOfNode;
		cin >> value;
		head = new ListNode(value);
		ListNode* cur = head;
		for (int i = 1; i < numOfNode; ++i){
			cin >> value;
			ListNode* temp = new ListNode(value);
			cur->next = temp;
			cur = temp;
		}
		cur->next = head->next;
		return head;
	}
	void printNode(ListNode* head){
		ListNode* cur = head;
		while (cur){
			cout << cur->val << " ";
			cur = cur->next;
		}
		cout << endl;
	}
};

int main(){
	ListNode* head = NULL;
	Solution solution;
	head = solution.createList(head);
	//solution.printNode(head);

	cout << solution.detectCycle(head)->val << endl;

	system("pause");
	return 0;
}

参考：http://blog.sina.com.cn/s/blog_725dd1010100tqwp.html