两个链表的第一个公共结点

//思路1
class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        set<ListNode*> filter;
        ListNode* res = NULL;
        while(pHead1 != NULL){
            filter.insert(pHead1);
            pHead1 = pHead1->next;
        }
        while(pHead2 != NULL){
            if(filter.find(pHead2) != filter.end()){
                res = pHead2;
                break;
            }
            pHead2 = pHead2->next;
        }
        return res;
    }
};

//思路二

class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        ListNode*p1, *p2;
        p1 = pHead1;
        p2 = pHead2;
        while(p1 != p2){
            if(p1 != NULL)
                p1 = p1->next;
            if(p2 != NULL)
                p2 = p2->next;
            if(p1 != p2){
                if(p1 == NULL) p1 = pHead2;
                if(p2 == NULL) p2 = pHead1;
            }
        }
        return p1;
    }
};