很妙很妙！但是一开始想不出来

/*
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 == nullptr || pHead2 == nullptr) {
        return nullptr;
      }
      
      ListNode *ptr1 = pHead1, *ptr2 = pHead2;
      
      while (ptr1 != ptr2) {
        // 两链表总长度之和是相等的！
        ptr1 = ptr1 ? ptr1->next : pHead2;
        ptr2 = ptr2 ? ptr2->next : pHead1;
      }
      
      return ptr1;
    }
};