// Time:  O(m + n)
// Space: O(1)

/**
 * 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 *curA = headA, *curB = headB;
        ListNode *begin = nullptr, *tailA = nullptr, *tailB = nullptr;
        while (curA && curB) {
            if (curA == curB) {
                begin = curA;
                break;
            }
   
            if (curA->next) {
                curA = curA->next;
            } else if (!tailA) {
                tailA = curA;
                curA = headB;
            } else {
                break;
            }

            if (curB->next) {
                curB = curB->next;
            } else if (!tailB) {
                tailB = curB;
                curB = headA;
            } else {
                break;
            }
        }
        
        return begin;
    }
};