（LeetCode）两个链表的第一个公共节点

LeetCode上面的题目如下：

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.

我的解决方法如下:

/**
 * 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) {
        
int lengthA = 0,lengthB = 0;
for(ListNode head= headA;head!=null;head = head.next)
{
lengthA++;
}
for(ListNode head = headB;head!=null;head = head.next)
{
lengthB++;
}
if(lengthA>=lengthB)
{
for(int i=0;i<lengthA-lengthB;i++)
{
headA = headA.next;
}
}
else
{
for(int i=0;i<lengthB-lengthA;i++)
{
headB = headB.next;
}
}
for(ListNode newA =headA,newB = headB;newA!=null&&newB!=null;newA = newA.next,newB = newB.next)
{
if(newA==newB)
{
return newA;
}
}
return null;
    }
}