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‎60-69/62. Intersection of Two Linked Lists(Easy)/README.md‎

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给你两个单链表的头节点 `headA` 和 `headB` ,请你找出并返回两个单链表相交的起始节点。如果两个链表没有交点,返回 `null` 。
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图示两个链表在节点 `c1` 开始相交:
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![160_statement.png](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)/160_statement.png)
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题目数据 保证 整个链式结构中不存在环。
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Given the heads of two singly linked-lists `headA` and `headB`, return the node at which the two lists intersect. If the two linked lists have no intersection at all, return `null`.
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For example, the following two linked lists begin to intersect at node `c1`:
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![160_statement.png](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)/160_statement.png)
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The test cases are generated such that there are no cycles anywhere in the entire linked structure.
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**example 1**
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![160_example_1.png](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)/160_example_1.png)
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```
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Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,6,1,8,4,5], skipA = 2, skipB = 3
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Output: Intersected at '8'
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**example 2**
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![160_example_2.png](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)/160_example_2.png)
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```
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Input: intersectVal = 2, listA = [1,9,1,2,4], listB = [3,2,4], skipA = 3, skipB = 1
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Output: Intersected at '2'
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```
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**example 3**
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![160_example_3.png](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)/160_example_3.png)
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```
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Input: intersectVal = 0, listA = [2,6,4], listB = [1,5], skipA = 3, skipB = 2
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Output: No intersection
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1. 解决这个问题的关键是,通过某些方式,让 `p1` 和 `p2` 能够同时到达相交节点 `c1`
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2. 我们可以让 `p1` 遍历完链表 `A` 之后开始遍历链表 `B`,让 `p2` 遍历完链表 `B` 之后开始遍历链表 `A`,这样相当于「逻辑上」两条链表接在了一起。如果这样进行拼接,就可以让 `p1` 和 `p2` 同时进入公共部分,也就是同时到达相交节点 `c1`
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3. 具体原理如下图
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![img](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)/2.jpeg)
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/*
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* @lc app=leetcode id=876 lang=cpp
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*
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* [876] Middle of the Linked List
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*/
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// @lc code=start
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/**
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* Definition for singly-linked list.
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* struct ListNode {
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* int val;
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* ListNode *next;
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* ListNode() : val(0), next(nullptr) {}
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* ListNode(int x) : val(x), next(nullptr) {}
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* ListNode(int x, ListNode *next) : val(x), next(next) {}
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* };
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*/
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class Solution {
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public:
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ListNode* middleNode(ListNode* head) {
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if (head == nullptr)
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return head;
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ListNode *fast, *slow;
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fast = slow = head;
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while (fast->next != nullptr && fast->next->next != nullptr)
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{
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fast = fast->next->next;
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slow = slow->next;
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}
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// 偶数个
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if (fast->next != nullptr && fast->next->next == nullptr)
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return slow->next;
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else
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return slow;
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}
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};
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// @lc code=end
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# 链表的中间结点
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#### *Middle of the Linked List*
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给定一个头结点为 head 的非空单链表,返回链表的中间结点。
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如果有两个中间结点,则返回第二个中间结点。
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提示:
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给定链表的结点数介于 1 和 100 之间。
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英文题目:
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Given the head of a singly linked list, return the middle node of the linked list.
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If there are two middle nodes, return the `second middle` node.
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**example 1**
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![test 1](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/63.%20Middle%of%the%Linked%List(Easy)/lc-midlist1.jpg)
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```
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Input: head = [1,2,3,4,5]
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Output: [3,4,5]
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Explanation: The middle node of the list is node 3.
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```
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**example 2**
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![test 2](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/63.%20Middle%of%the%Linked%List(Easy)/lc-midlist2.jpg)
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```
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Input: head = [1,2,3,4,5,6]
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Output: [4,5,6]
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Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.
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```
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**Constraints:**
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The number of nodes in the list is in the range `[1, 100]`.
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1 <= `Node.val` <= 100
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---
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### 思路
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1. 使用快慢两个指针,快指针每次走2步,慢指针每次走1步,当快指针到达链表尾部时,慢指针指向链表中点
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2. 注意处理链表长度为偶数的情况
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### 代码
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```
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/*
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* @lc app=leetcode id=876 lang=cpp
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*
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* [876] Middle of the Linked List
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*/
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// @lc code=start
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/**
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* Definition for singly-linked list.
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* struct ListNode {
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* int val;
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* ListNode *next;
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* ListNode() : val(0), next(nullptr) {}
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* ListNode(int x) : val(x), next(nullptr) {}
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* ListNode(int x, ListNode *next) : val(x), next(next) {}
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* };
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*/
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class Solution {
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public:
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ListNode* middleNode(ListNode* head) {
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if (head == nullptr)
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return head;
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ListNode *fast, *slow;
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fast = slow = head;
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while (fast->next != nullptr && fast->next->next != nullptr)
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{
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fast = fast->next->next;
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slow = slow->next;
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}
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// 偶数个
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if (fast->next != nullptr && fast->next->next == nullptr)
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return slow->next;
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else
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return slow;
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}
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};
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// @lc code=end
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```
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本题以及其它leetcode题目代码github地址: [github地址](https:github.com/SherlockUnknowEn/leetcode)
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‎README.md‎

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##### 58. [79.Word Search](https://github.com/SherlockUnknowEn/leetcode/tree/master/50-59/58.%20Word%20Search(Medium)) 单词搜索
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##### 59. [80.Remove Duplicates from Sorted Array II](https://github.com/SherlockUnknowEn/leetcode/tree/master/50-59/59.%20Remove%20Duplicates%20from%20Sorted%20Array%20II(Medium)) 删除排序数组中的重复项 II
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##### 60. [141.Linked List Cycle](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/60.%20Linked%20List%20Cycle(Easy)) 环形链表
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##### 61. [142.Linked List Cycle II](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/61.%20Linked%20List%20Cycle%20II(Medium)) 环形链表2
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##### 62. [160.Intersection of Two Linked Lists](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)) 环形链表2
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##### 61. [142.Linked List Cycle II](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/61.%20Linked%20List%20Cycle%20II(Medium)) 环形链表 II
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##### 62. [160.Intersection of Two Linked Lists](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)) 相交链表
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##### 63. [876.Middle of the Linked List](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/63.%20Middle%of%the%Linked%List(Easy)) 链表的中间结点

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