/*

 找出2个链表的长度，然后让长的先走两个链表的长度差，然后再一起走

 （因为2个链表用公共的尾部）

 */

 class Solution {

 public:

     ListNode* FindFirstCommonNode( ListNode *pHead1, ListNode *pHead2) {

         int len1 = findListLenth(pHead1);

         int len2 = findListLenth(pHead2);

         if(len1 > len2){

             pHead1 = walkStep(pHead1,len1 - len2);

         }else{

             pHead2 = walkStep(pHead2,len2 - len1);

         }

         while(pHead1 != NULL){

             if(pHead1 == pHead2) return pHead1;

             pHead1 = pHead1->next;

             pHead2 = pHead2->next;

         }

         return NULL;

     }

      

     int findListLenth(ListNode *pHead1){

         if(pHead1 == NULL) return 0;

         int sum = 1;

         while(pHead1 = pHead1->next) sum++;

         return sum;

     }

      

     ListNode* walkStep(ListNode *pHead1, int step){

         while(step--){

             pHead1 = pHead1->next;

         }

         return pHead1;

     }

 };

 class Solution {

 public:

     ListNode* FindFirstCommonNode( ListNode *pHead1, ListNode *pHead2) {

         ListNode *p1 = pHead1;

         ListNode *p2 = pHead2;

         while(p1!=p2){

             p1 = (p1==NULL ? pHead2 : p1->next);

             p2 = (p2==NULL ? pHead1 : p2->next);

         }

         return p1;

     }

 };