leetcode--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.

解：

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */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;        //如果某一条链表到达尾部，将它指向另一条链表的头        //这样两个指针指向相交节点时走了同样的路程        //如果两个链表不想交，两个指针最终同时指向尾部NULL返回        if (p1 == NULL) p1 = headB;        if (p2 == NULL) p2 = headA;    }    return p1;}};