Intersection of Two Linked Lists【找链表的汇合点】

PROBLEM:

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.

SOLVE:

C++：

ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) 
{
    ListNode *p1 = headA;
    ListNode *p2 = headB;     
    if (p1 == NULL || p2 == NULL) return NULL;
    while (p1 != p2) {
        p1 = (p1 == NULL? headB : p1->next);
        p2 = (p2 == NULL? headA : p2->next);
    }    return p1;
}

Python：

class Solution:
    def getIntersectionNode(self, headA, headB):
        if headA is None or headB is None:
            return None
        pa = headA
        pb = headB
        while pa is not pb:
            pa = headB if pa is None else pa.next
            pb = headA if pb is None else pb.next
        return pa

解释：每一个指针都把两个链表跑一遍，最后结果要么都是指向NULL要么相同。