哈希map

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        unordered_map<ListNode *, int> m;
        //将pHead1的节点全部放入map中
        while(pHead1){
            m[pHead1]++;
            pHead1 = pHead1->next;
        }
        while(pHead2){
            m[pHead2]++;
            if(m[pHead2]==2) return pHead2;//如果存在，则返回
            pHead2 = pHead2->next;
        }
        return pHead2;
    }
};