[LeetCode] Intersection of Two Linked Lists

Question：

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.

1、题型分类：

2、思路：

3、时间复杂度：O(n)

4、代码：

空间复杂度O(n)

public class Solution {

    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {

        if(headA==null || headB==null) return null;

        Set<ListNode> set=new HashSet<ListNode>();

        while(headA!=null)

        {

            if(set.contains(headA)) return headA;

            else set.add(headA);

            headA=headA.next;

        }

        while(headB!=null)

        {

            if(set.contains(headB)) return headB;

            else set.add(headB);

            headB=headB.next;

        }

        return null;

    }

}

 

5、优化：

先计算两个链表的长度，然后分别从相同长度的节点开始遍历，遇到相同的元素返回。

public class Solution {

    

    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {

        int lenA = getLen(headA);

        int lenB = getLen(headB);

        

        ListNode shortHead = headA;

        ListNode longHead = headB;

        if (lenA > lenB) {

            shortHead = headB;

            longHead = headA;

        }

        

        int i = 0;

        while (i < Math.abs(lenB - lenA)) {

            i++;

            longHead = longHead.next;

        }

        

        while (shortHead != longHead) {

            shortHead = shortHead.next;

            longHead = longHead.next;

            

            

            if (shortHead == null) {

                return null;

            }

        }

        return shortHead;

    }

    

    private int getLen(ListNode head) {

        int len = 0;

        while (head != null) {

            len++;

            head = head.next;

        }

        return len;

    }

}

 

6、扩展：