LeetCode OJ 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.
这个题目是要找出两个链表的交叉点,该如何解决呢?如果两个链表相交,我们从A链表出发移动一段距离alen,出B列表出发移动一段距离blen,那么会发现他们指向同一个节点c1。那么这个距离是多少呢?
我们把每一个链表看成两段,不相交的一段和相交的一段。相交的一段对于两个链表长度是一样的,不想交的一段链表的长度是不同的。如果我们分别计算出两个链表的长度,然后计算他们长度的差值f,然后在较长的链表上先移动距离f,再同时从A,B链表出发开始遍历,若发现他们指向同一个节点,则他们相交并返回相交的点,若他们不想交,则会遍历到链表的尾部,则返回null。代码如下:
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 ListNode p1 = headA, p2 = headB; 15 int len1 = 0, len2 = 0; 16 while (p1 != null) { //求链表A的长度 17 p1 = p1.next; 18 len1++; 19 } 20 while (p2 != null) { //求链表B的长度 21 p2 = p2.next; 22 len2++; 23 } 24 p1 = headA; 25 p2 = headB; 26 if (len1 > len2) { //计算链表长度的差值并在较长的链表上向后移动|len1-len2| 27 for (int i = 0;i < len1 - len2; i++) { 28 p1 = p1.next; 29 } 30 } else { 31 for (int i = 0;i < len2 - len1; i++) { 32 p2 = p2.next; 33 } 34 } 35 while (p1 != p2) { //向后遍历链表A和链表B,找到相交的节点,若遍历到最后,返回null 36 p1 = p1.next; 37 p2 = p2.next; 38 } 39 return p1; 40 } 41 }