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.

思路：遍历其中一个链表，用一个哈希集合存储这个链表的所有节点，再遍历另一条，看哪个节点在集合中。

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
    if (headA == null || headB == null) {
        return null;
    }

    HashSet<ListNode> set = new HashSet<>();
    while (headA != null) {
        set.add(headA);
        headA = headA.next;
    }

    while (headB != null) {
        if (set.contains(headB)) {
            return headB;
        }
        headB = headB.next;
    }

    return null;
}