輸入兩個鏈表，找出它們的第一個公共結點

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        ListNode * p = pHead1;
        int num1 = 0;
        int num2 = 0;
        while (p)
            {
                num1++;
                p = p->next;
        }
        ListNode *p2 = pHead2;
        while(p2)
            {
                num2++;
                p2 = p2->next;
        }
        if (num2>num1)
            {
                int x = num2-num1;
                for ( int i = 0 ; i!=x ; i++)
                    {
                        pHead2=pHead2->next;
                }
                while (pHead2)
                    {
                        if (pHead2->val == pHead1->val)
                            return pHead2;
                        pHead2 = pHead2->next;
                        pHead1 = pHead1->next;
                }
        }else{
            int x = num1 - num2;
            for (int i = 0; i!= x;i++)
                {
                pHead1 = pHead1->next;
            }
            while (pHead1)
                {
                if (pHead1->val == pHead2->val)
                    return pHead1;
                pHead1 = pHead1->next;
                pHead2 = pHead2->next;
            }
        }
        return NULL;
    }


};