Intersection of Two Linked Lists

Write a program to find the node at which the intersection of two singly linked lists begins.

 

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

 

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

 

/**
 * 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 *p1 = headA;
        ListNode *p2 = headB;
    
        if (p1 == NULL || p2 == NULL) return NULL;
        while (p1 != NULL && p2 != NULL && p1 != p2) {
            p1 = p1->next;
            p2 = p2->next;
            if (p1 == p2) return p1;
            if (p1 == NULL) p1 = headB;
            if (p2 == NULL) p2 = headA;
        }
    
        return p1;
    }
};