C++/代码：
链表中环的入口结点
http://www.nowcoder.com/questionTerminal/253d2c59ec3e4bc68da16833f79a38e4
图片：
C++/代码：
class Solution {
public:
ListNode* EntryNodeOfLoop(ListNode* pHead) {
auto *i = pHead, *j = pHead;
do {
if (!j->next) return NULL; //
i = i->next,j = j->next->next; //慢指针走一步，快指针走两步
}while(i != j);
for (i = pHead; i != j; i = i->next) j = j->next;
return i;
}
};
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