Intersection of Two Linked Lists（两个链表的第一个公共节点）

来源：https://leetcode.com/problems/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.

两个指针分别从两个头节点出发，若走完一个链表再从另一个链表的头节点继续前进，这样两个指针一定会经过同一个节点（若没有公共节点则最后都为null），就是第一个公共节点。

/**
 * 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) {
        ListNode p1 = headA, p2 = headB;
        while(p1 != p2) {
            p1 = (p1 == null) ? headB : p1.next;
            p2 = (p2 == null) ? headA : p2.next;
        }
        return p1;
    }
}