lec_25.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) {
            while(headB != NULL){
                ListNode * temp = headA ;
                while(temp!= NULL){
    
                    if(temp == headB) return headB;
                    
                    temp = temp->next;
                }
    
                headB = headB->next;
            }
    
            return NULL;
        }
    };


// alternative
class Solution {
    public:
        ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
    
            unordered_set<ListNode*>st;
    
            while(headA != NULL){
               st.insert(headA);
               headA = headA->next;
            }
            while(headB!=NULL){
                if(st.find(headB)!= st.end()) return headB;
                headB = headB->next;
            }
    
            return NULL;
        }
    };

// alternative