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; //當達到相遇點時，直接傳回p1或者p2

        if (p1 == NULL) p1 = headB; //當p1比p2先到達結尾時，重新将其移動到第二個連結清單的頭部
        if (p2 == NULL) p2 = headA; //當p2比p1先到達結尾時，重新将其移動到第二個連結清單的頭部，這樣的話，二者到相遇點的距離是相等的
    }

    return p1;
   }
};