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.

code: HashSet

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {

        //build a hash map for headA
        Set<ListNode> hashset = new HashSet<ListNode>();

        //corner case
        if(headA == null || headB == null) return null;



        //build a hash set for headA
        ListNode point = headA;
        while(point !=null) {
            hashset.add(point);
            point = point.next;
        }

        //check intersaction
        point = headB;
        while(point != null) {
            if(hashset.contains(point)) {
                return point;
            }
            point = point.next;
        }

        return null;
    }

复杂度：时间O(N), 空间O(N)

Solution: O(N)time & O(1)space

思路：

让指针同时移动，同时到达intersaction