双指针：遍历两次
a+重+b=b+重+a

/**
 * 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) {
        if(headA==null||headB==null){
            return null;
        }
        ListNode a=headA,b=headB;
        while(a!=b){
            a = a== null? headB: a.next;
            b = b== null? headA: b.next;
        }
        return a;
    }
}

遍历一次

/**
 * 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 lena=0,lenb=0,dif=0;
        ListNode a=headA,b=headB;
        while(a!=null){
            lena++;
            a=a.next;
        }
        while(b!=null){
            lenb++;
            b=b.next;
        }
        if(lena<lenb){
            a=headB;
            b=headA;
            dif=lenb-lena;
        }else{
            a=headA;
            b=headB;
            dif=lena-lenb;
        }
        for(int i=0;i<dif;i++){
            a=a.next;
        }
        while(a!=null&&b!=null){
            if(a==b){
                return a;
            }
            a=a.next;
            b=b.next;
        }
        return null;
    }
}