LeetCode 160 Intersection of Two Linked Lists

原题

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.

解题思路

思路一：

1. 求出A和B两个链表的长度，取两者长度差，因为相等部分的长度肯定是相同的，所以将较长的链表遍历长度差，之后同时遍历A和B即可。

思路二：

1. 思路一的升级版，非常巧妙的一个算法，虽然A和B的长度不一定相等，但是A+B的长度是一样的
2. 所以就利用了在A链表后连B链表的方法（B同理）
3. 这样在第二次迭代遍历时就消除了长度差，如果有相同节点则为答案
4. 如果没有相同节点，那么A和B的末尾就是None相等，返回None

完整代码

思路一：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        A = headA
        B = headB
        lengthA = self.lengthLink(A)
        lengthB = self.lengthLink(B)
        value = lengthA - lengthB
        
        for i in xrange(abs(value)):
            if value >= 0:
                A = A.next
            else:
                B = B.next
                
        while A != B:
            A = A.next
            B = B.next
            
        return A
        
        
    def lengthLink(self, node):
        length = 0
        while node:
            length += 1
            node = node.next
        return length

　　

思路二：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        if headA == None or headB == None:
            return None
        
        a = headA
        b = headB
        # if a & b have different len, then we will stop the loop after second iteration
        while a!=b:
            # for the end of first iteration, we just reset the pointer to the head of another linkedlist
            a = headB if a == None else a.next
            b = headA if b == None else b.next
            
        return a