160. Intersection of Two Linked Lists_spaceConstant.cc

//TC:O(m + n)
//SC: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* ptrA = headA;
        ListNode* ptrB = headB;
        // ptrA: [A + common + B] + common
        // ptrB: [B + common + A] + common
        while (ptrA != ptrB) {
            ptrA = ptrA ? ptrA->next : headB;
            ptrB = ptrB ? ptrB->next : headA;
        }
        
        return ptrA;
    }
};