题解 | #两个链表的第一个公共结点#

双指针交换循环需要加一个计数器，否则可能死循环

/*
struct ListNode {
	int val;
	struct ListNode *next;
	ListNode(int x) :
			val(x), next(NULL) {
	}
};*/
class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
		if(!pHead1 || !pHead2) return NULL;
        ListNode *p1 = pHead1, *p2 = pHead2;
		int cnt = 0;
		while(p1 != p2) {
			p1 = p1->next;
			p2 = p2->next;
			if(p1 == p2) return p1;
			if(!p1) {
				p1 = pHead2;
				cnt++;
			}
			if(!p2) p2 = pHead1;

			if(cnt == 2) return NULL;
		}
		return p1;

    }
};