https://leetcode.com/problems/intersection-of-two-linked-lists/

160-Intersection-of-Two-Linked-Lists.cpp

/**
 * 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) {
        if(!headA or !headB) return NULL;

        auto pA = headA;
        auto pB = headB;
                
        while(pA != pB){
            // move both 1 step
            pA = pA->next;
            pB = pB->next;
            
            // meeth same node
            if(pA == pB) break;
            
            // restart different list
            if(pA == NULL) pA = headB;
            if(pB == NULL) pB = headA;
        }

        return pA;
    }
};