双指针，p指针先遍历A再遍历B，q指针先遍历B再遍历A，如果相交则一定会有p==q
public class Solution {
public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
if(headA==null || headB==null) return null;
ListNode p = headA;
ListNode q = headB;
if(p == q) return p;
while(p!=null && q!=null) {
p = p.next;
q = q.next;
}
if(p==null) p = headB; else q = headA;
while(p!=null && q!=null) {
p = p.next;
q = q.next;
}
if(p==null) p = headB; else q = headA;
while(p!=null && q!=null) {
if(p==q) return p;
p = p.next;
q = q.next;
}
return null;
}
}
