[LeetCode] Intersection of Two Linked Lists 两链表是否相交

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.

 

Credits:
Special thanks to @stellari for adding this problem and creating all test cases.

 

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 Linked List

 

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     两个单项链表，判断是否存在交集，如上图很清晰，最直观的方法是

for  list1 begin to last

　　for list2 begin to last

　　　　if list2==list1 success

　　end

end　　

    时间是O(nm)，空间挺少的O(1)。如何提高呢？

1. 遍历list1 ，将其节点存在hash_table
2. 遍历list2，如果已经在hash_table中，那么存在

    利用hash_table 可以提升时间到O(n+m)，可是空间变O(n)了

 1 class Solution {
 2 public:
 3     ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
 4         unordered_map<ListNode*,int> m;
 5         while(headA!=NULL){
 6             m[headA] = 1;
 7             headA=headA->next;
 8         }
 9         while(headB!=NULL){
10             if(m[headB]==1) return headB;
11             headB=headB->next;
12         }
13         return NULL;
14     }
15 };

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