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.

代码如下：

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) {
  *         val = x;
  *         next = null;
  *     }
  * }
  */
 public class Solution {
     public ListNode getIntersectionNode(ListNode headA, ListNode headB) {

         if(headA==null||headB==null)
         return null;
         Map<ListNode,ListNode> map=new <ListNode,ListNode>HashMap();

         while(headA!=null||headB!=null)
         {

             if(map.containsKey(headA)&&headA!=null)
             return headA;
             else
             if(!map.containsKey(headA)&&headA!=null)
             {
                 map.put(headA,headA);
                 headA=headA.next;
             }

         if(map.containsKey(headB)&&headB!=null)
         return headB;
         else
         if(!map.containsKey(headB)&&headB!=null)
         {
         map.put(headB,headB);
         headB=headB.next;
         }

         }

         return null;
     }
 }