Intersection of Two Linked Lists（leetcode160）

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.

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

    ListNode a = headA;
    ListNode b = headB;

    //这么写还是有点趣的，因为查完A list然后接着是 Blist，那么这样就可以不用考虑长度不等啦
    while( a != b){
        a = a == null? headB : a.next;
        b = b == null? headA : b.next;
    }

    return a;
}

public ListNode getIntersectionNode2(ListNode headA, ListNode headB) {

    if(headA==null || headB==null) {
        return null;
    }

    Set<ListNode> set = new HashSet<ListNode>();

    ListNode a = headA;
    ListNode b = headB;

    while(a!=null) {
        set.add(a);
        a=a.next;
    }
    while(b!=null)  {
        if(set.contains(b)) {
            return b;
        }
        b = b.next;
    }
    return null;
}