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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        
        ListNode* pa = headA;
        ListNode* pb = headB;
        
        if(!pa||!pb) return nullptr;
        while(true){
            if(pa == pb) {
                return pa;
                break;
            }
            
              //  cout << "pa:" << pa->val << ",";
              //  cout << "pb:" << pb->val << ",";
                pa = pa->next;
                pb = pb->next;
                if(!pa||!pb){
                    break;
                }
              
        }
        
        if(!pa&&!pb) return nullptr;
        
    if(!pa) {
        pa = headB;
        while(pa){
            if(pa == pb){
                return pa;
            }
            
            pa = pa->next;
            pb = pb->next;
            if(!pb){
                pb = headA;
            }
        }
        
    } else if(!pb){
        pb = headA;
        while(pb){
            if(pa == pb){
                return pa;
            }
            
            pa = pa->next;
            pb = pb->next;
            if(!pa){
                pa  = headB;
            }
        }
    }
    return nullptr;        
    }
};