链表

两个链表的长度如果相等的话，从头节点开始走，两个指针相等的时候就找到了第一个公共节点。

class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        ListNode* L1 = pHead1;
        ListNode* L2 = pHead2;
        int l1 = 0,l2 = 0;
        while(L1 !=NULL || L2!= NULL)
        {
            if(L1!=NULL){L1 = L1->next;}
            else{ l2++; }
            if(L2 != NULL){L2 = L2->next;}
            else{ l1++; }
        }
        L1 = pHead1;
        L2 = pHead2;
        while(l1!=0)
        {
            L1 = L1->next;
            l1--;
        }
        while(l2!=0)
        {
            L2 = L2->next;
            l2--;
        }
        while(L1!=L2)
        {
            L1 = L1->next;
            L2 = L2->next;
        }
        return L1;
    }
};