一图胜千言，java 代码如下：

/**
 * 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) {
        if(headA == null || headB == null) {
            return null;
        }
        ListNode pa = headA;
        ListNode pb = headB;
        while(pa != pb) {
            pa = (pa == null) ? headB : pa.next; //(pa == null)判断是否到达了末尾，是则从pb的头节点开始，否则继续往后走
            pb = (pb == null) ? headA : pb.next; //pa走一步之后，pb再走，这两行代码的顺序不影响结果。
        }
        return pa;
    }
}