160. 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.

S1: hash table

S2: 将l2首尾相连，从l1开始找环的起始，和Linked List Cycle方法一样

S3: 用两个指针p1,p2, p1遍历l1->l2，p2遍历l2->l1直到p1 = p2 两支针相遇点为intersection或NULL(无intersection,到达链表尾)

    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode *p1 = headA, *p2 = headB;
        while((p1 || p2) && p1 != p2){
            p1 = p1? p1->next: headB;
            p2 = p2? p2->next: headA;
        }
        return p1;
    }