Leetcode 160：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.

class Solution {
public:
   ListNode *getIntersectionNode(ListNode *headA, ListNode *headB)
   {
    ListNode *p1 = headA;
    ListNode *p2 = headB;

    if (p1 == NULL || p2 == NULL) return NULL;

    while (p1 != NULL && p2 != NULL && p1 != p2) 
    {
        p1 = p1->next;
        p2 = p2->next;

        if (p1 == p2) return p1; //当达到相遇点时，直接返回p1或者p2

        if (p1 == NULL) p1 = headB; //当p1比p2先到达结尾时，重新将其移动到第二个链表的头部
        if (p2 == NULL) p2 = headA; //当p2比p1先到达结尾时，重新将其移动到第二个链表的头部，这样的话，二者到相遇点的距离是相等的
    }

    return p1;
   }
};