面试题56 链表中环的入口结点

题目描述

/*
struct ListNode {
int val;
struct ListNode *next;
ListNode(int x) :
val(x), next(NULL) {
}
};
*/
class Solution {
public:
ListNode* EntryNodeOfLoop(ListNode* pHead)
{
if (!pHead || !pHead->next)
return NULL;
ListNode *p = pHead;
ListNode *q = pHead;
while(p != NULL && q->next != NULL ){
p = p->next;
q = q->next->next;
if(p == q){
q = pHead;
while(p != q){
p = p->next;
q = q->next;
}
if(p == q)
return p;
}
}
return NULL;
}
};