Cracking The Coding Interview 2.5

这题的思想来自于http://hawstein.com/posts/2.5.html，重新实现了一下

用hash来记录循环的起点

//Given a circular linked list, implement an algorithm which returns node at the beginning of the loop.

//DEFINITION

//Circular linked list: A (corrupt) linked list in which a node’s next pointer points to an earlier node, so as to make a loop in the linked list.

//EXAMPLE

//Input: A -> B -> C -> D -> E -> C [the same C as earlier]

#include <iostream>
#include <map>
using namespace std;

typedef struct node{
	int data;
	node *next;
}node;

node* init(int a[], int n, int m){
	node *head, *p, *q;
	for(int i=0; i<n; ++i){
		node *nd = new node();
		nd->data = a[i];
		if(i==m) q = nd;
		if(i==0){
			head = p = nd;
			continue;
		}
		p->next = nd;
		p = nd;
	}
	p->next = q;
	return head;
}

map<node *, bool> st;
node * getLoopStart(node *head)
{
	node *p =head->next;
	while(p)
	{
		if (st[p] != true)
		{
			st[p] = true;
			p=p->next;
		}
		else
			return p;
	}
	return p;
}

int main(){
	int n = 10, m = 9;// m<n
	int a[] =
	{
		3, 2, 1, 3, 5, 6, 2, 6, 3, 1
	};
	node *head = init(a, n, m);
	//node *p = loopstart(head);

	node *p2 = getLoopStart(head);
	if(p2)
		cout<<p2->data<<endl;
	return 0;
}