注意一个坑，就算两个链表不想交，经过while循环后，p1和p2都会指向nullptr

此时也算p1 == p2

/*
struct ListNode {
	int val;
	struct ListNode *next;
	ListNode(int x) :
			val(x), next(NULL) {
	}
};*/
class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        // 平行指针
        ListNode* p1 = pHead1;
        ListNode* p2 = pHead2;
        
        while(p1 != p2){
            p1 = p1 ? p1->next : pHead2;
            p2 = p2 ? p2->next : pHead1;
        }
        return p1;
    }
};