(java)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.

思路:本题是求两个串的最长后缀子串。判断当前相同的位置是不是相同子串中第一位，如果是记录下当前的位置，如果不是则一直向前遍历，如果不相同，则重新开始记
代码如下(已通过leetcode)
public class Solution {

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {

ListNode p=headA;

ListNode q=headB;

int length1=0;

int length2=0;

boolean isfirst=true;

while(p!=null) {length1++;p=p.next;}

while(q!=null) {length2++;q=q.next;}

int a=Math.abs(length1-length2);

if(length1>length2) {

for(int i=0;i<a;i++) headA=headA.next;

} else {

for(int i=0;i<a;i++) headB=headB.next;

}

ListNode flag=null;

ListNode ans=null;

while(headA!=null) {

if(headA.val!=headB.val) {

flag=null;

ans=null;

isfirst=true;

} else {

if(isfirst) {

ans=headA;

isfirst=false;

}

flag=headA;

flag=flag.next;

}

headA=headA.next;

headB=headB.next;

}

return ans;

}

}