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.

 

Credits:
Special thanks to @stellari for adding this problem and creating all test cases.

这道题是说寻找两个链表的公共部分的起始节点，有这么几个方法：

1、暴力搜索

时间复杂度太高

2、使用hash表

空间复杂度不满足要求

3、双指针

方法非常巧妙，设置两个指针分别从两个链表头部开始遍历，如果某个指针到达尾部，则从另一个链表头部开始，最后两个指针相遇的地方就是公共节点

/**
 * 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) {
        if (!headA || !headB)
            return nullptr;
        ListNode *a = headA, *b = headB;
        while (a != b)
        {
            a = a ? a->next : headB;
            b = b ? b->next : headA;
        }
        return a;
    }
};

时间复杂度：O(m+n)

空间复杂度：O(1)

参考：https://leetcode.com/problems/intersection-of-two-linked-lists/solution/

https://leetcode.com/problems/intersection-of-two-linked-lists/discuss/49799