灵活运用链表长度加减法，对链表分段相加长度相同

a+b+c = c+b+a

双指针指向相同即使共同部分b的入口节点，a+b到尾部则跳转另一链表c+b，同理，c+b到尾部则跳转a+b。

class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
		ListNode* first = pHead1;
		ListNode* second = pHead2;
		while(first != second){
			first = (first==nullptr?pHead2:first->next);
			second = (second==nullptr?pHead1:second->next);
		}
		return first;
    }
};