160. Intersection of Two Linked Lists，intersectionlinked

160. Intersection of Two Linked Lists，intersectionlinked

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.

代碼如下：

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
14         
15         if(headA==null||headB==null)
16         return null;
17         Map<ListNode,ListNode> map=new <ListNode,ListNode>HashMap();
18         
19         while(headA!=null||headB!=null)
20         {
21             
22             if(map.containsKey(headA)&&headA!=null)
23             return headA;
24             else 
25             if(!map.containsKey(headA)&&headA!=null)
26             {
27                 map.put(headA,headA);
28                 headA=headA.next;
29             }
30        
31         
32         if(map.containsKey(headB)&&headB!=null)
33         return headB;
34         else 
35         if(!map.containsKey(headB)&&headB!=null)
36         {
37         map.put(headB,headB);
38         headB=headB.next;
39         }
40             
41         }
42         
43         return null;
44     }
45 }