Write a program to find the node at which the intersection of two singly linked lists begins.

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null) return null;
        ListNode A = headA, B = headB;//must create two new node, can't change headA headB
        while (A != B) {
            A = A != null ? A.next : headB;// A = A.next = null ?....wrong!!
            B = B != null ? B.next : headA;
            //必须走到null不然就在headA前加上了node形成无限循环
            //if there is no intersection, the loop stops when both A and B gets to null
            //the length of two list should be the same now!
        }
        return A;
    }
}

class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """

        if not headA or not headB:
            return

        A, B = headA, headB
        while A != B:
            A = A.next if A else headB
            B = B.next if B else headA
        return A

/**
 * 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* dummyA = headA;
        ListNode* dummyB = headB;
        while (dummyA != dummyB) {
            if (dummyA != NULL) {
                dummyA = dummyA->next;
            } else {
                dummyA = headB;
            }
            if (dummyB != NULL) {
                dummyB = dummyB->next;
            } else {
                dummyB = headA;
            }
        }
        return dummyA;
    }
};