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.

解法:

1.p1遍历链表A,p2遍历链表B

2.如果p1 到达了链表尾部p1开始遍历链表B

3.如果p2 到达了链表尾部,p2开始遍历链表A

4.如果p1 == p2 则找到了分叉点

代码:

class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        if headA ==None or headB ==None:
            return None
        p1=headA
        p2=headB
        while p1 is not p2:
            p1 = p1.next
            p2 = p2.next
            if p1 is p2:
                return p1
            if p1 is None:
                p1 = headB
            if p2 is None:
                p2 = headA
        return p1