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;    }};

Intersection of Two Linked Lists