# 160. Intersection of Two Linked Lists

## 題目敘述

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`.

For example, the following two linked lists begin to intersect at node `c1`:

The test cases are generated such that there are no cycles anywhere in the entire linked structure.

Note that the linked lists must retain their original structure after the function returns.

Custom Judge:

The inputs to the judge are given as follows (your program is not given these inputs):

• `intersectVal` - The value of the node where the intersection occurs. This is `0` if there is no intersected node.
• `listA` - The first linked list.
• `listB` - The second linked list.
• `skipA` - The number of nodes to skip ahead in `listA` (starting from the head) to get to the intersected node.
• `skipB` - The number of nodes to skip ahead in `listB` (starting from the head) to get to the intersected node.

The judge will then create the linked structure based on these inputs and pass the two heads, `headA` and `headB` to your program. If you correctly return the intersected node, then your solution will be accepted.

Example 1:

``````Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,6,1,8,4,5], skipA = 2, skipB = 3
Output: Intersected at '8'
Explanation: The intersected node's value is 8 (note that this must not be 0 if the two lists intersect).
From the head of A, it reads as [4,1,8,4,5]. From the head of B, it reads as [5,6,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.
``````

Example 2:

``````Input: intersectVal = 2, listA = [1,9,1,2,4], listB = [3,2,4], skipA = 3, skipB = 1
Output: Intersected at '2'
Explanation: The intersected node's value is 2 (note that this must not be 0 if the two lists intersect).
From the head of A, it reads as [1,9,1,2,4]. From the head of B, it reads as [3,2,4]. There are 3 nodes before the intersected node in A; There are 1 node before the intersected node in B.
``````

Example 3:

``````Input: intersectVal = 0, listA = [2,6,4], listB = [1,5], skipA = 3, skipB = 2
Output: No intersection
Explanation: From the head of A, it reads as [2,6,4]. From the head of B, it reads as [1,5]. Since the two lists do not intersect, intersectVal must be 0, while skipA and skipB can be arbitrary values.
Explanation: The two lists do not intersect, so return null.
``````

Constraints:

• The number of nodes of `listA` is in the `m`.
• The number of nodes of `listB` is in the `n`.
• `1 <= m, n <= 3 * 10**4`
• `1 <= Node.val <= 10**5`
• `0 <= skipA < m`
• `0 <= skipB < n`
• `intersectVal` is `0` if `listA` and `listB` do not intersect.
• `intersectVal == listA[skipA] == listB[skipB]` if `listA` and `listB` intersect.

Follow up: Could you write a solution that runs in `O(m + n)` time and use only `O(1)` memory?

## 解法解析

### Two pointer

``````A = a + C
B = b + C
``````

``````Pointer A：(a + C) + (b + C)
Pointer B：(b + C) + (a + C)
``````

### 解法範例

#### Go

##### Brute force
``````/**
* Definition for singly-linked list.
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
func getIntersectionNode(headA, headB *ListNode) *ListNode {
for headA != nil {
var pB *ListNode = headB

for pB != nil {
if headA == pB {
return headA
}
pB = pB.Next
}
headA = headA.Next
}
return nil
}
``````
##### Hash table
``````/**
* Definition for singly-linked list.
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
func getIntersectionNode(headA, headB *ListNode) *ListNode {
var nodesInB = make(map[*ListNode]bool)

for headB != nil {
nodesInB[headB] = true
headB = headB.Next
}

for headA != nil {
if nodesInB[headA] {
return headA
}
headA = headA.Next
}
return nil
}
``````
##### Two pointer
``````/**
* Definition for singly-linked list.
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
func getIntersectionNode(headA, headB *ListNode) *ListNode {
var pA, pB *ListNode = headA, headB
for pA != pB {
if pA == nil {
pA = headB
} else {
pA = pA.Next
}
if pB == nil {
pB = headA
} else {
pB = pB.Next
}
}
return pA
}
``````

#### JavaScript

##### Brute force
``````/**
* Definition for singly-linked list.
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @param {ListNode} headA
* @param {ListNode} headB
* @return {ListNode}
*/
var getIntersectionNode = function (headA, headB) {
while (headA) {
let pB = headB;
while (pB) {
if (headA === pB) {
return headA;
}
pB = pB.next;
}
headA = headA.next;
}
return null;
};
``````
##### Hash table
``````/**
* Definition for singly-linked list.
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @param {ListNode} headA
* @param {ListNode} headB
* @return {ListNode}
*/
var getIntersectionNode = function (headA, headB) {
const nodesInB = new Set();

while (headB) {
nodesInB.add(headB);
headB = headB.next;
}
while (headA) {
if (nodesInB.has(headA)) {
return headA;
}
headA = headA.next;
}
return null;
};
``````
##### Two pointer
``````/**
* Definition for singly-linked list.
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @param {ListNode} headA
* @param {ListNode} headB
* @return {ListNode}
*/
var getIntersectionNode = function (headA, headB) {
let pA = headA;
let pB = headB;
while (pA !== pB) {
pA = pA ? pA.next : headB;
pB = pB ? pB.next : headA;
}
return pA;
};
``````

#### Kotlin

##### Brute force
``````/**
* Example:
* var li = ListNode(5)
* var v = li.`val`
* Definition for singly-linked list.
* class ListNode(var `val`: Int) {
*     var next: ListNode? = null
* }
*/

class Solution {
fun getIntersectionNode(headA:ListNode?, headB:ListNode?):ListNode? {
var pA: ListNode? = headA
while (pA != null) {
var pB: ListNode? = headB
while (pB != null) {
if (pB == pA) {
return pA
}
pB = pB.next
}
pA = pA.next
}
return null
}
}
``````
##### Hash table
``````/**
* Example:
* var li = ListNode(5)
* var v = li.`val`
* Definition for singly-linked list.
* class ListNode(var `val`: Int) {
*     var next: ListNode? = null
* }
*/

class Solution {
fun getIntersectionNode(headA:ListNode?, headB:ListNode?):ListNode? {
var nodesInB = HashSet<ListNode>()
var node: ListNode? = headB
while (node != null) {
nodesInB.add(node)
node = node.next
}

node = headA
while (node != null) {
if (nodesInB.contains(node)) {
return node
}
node = node.next
}
return null
}
}
``````
##### Two pointer
``````/**
* Example:
* var li = ListNode(5)
* var v = li.`val`
* Definition for singly-linked list.
* class ListNode(var `val`: Int) {
*     var next: ListNode? = null
* }
*/

class Solution {
fun getIntersectionNode(headA:ListNode?, headB:ListNode?):ListNode? {
var pA: ListNode? = headA
var pB: ListNode? = headB
while (pA != pB) {
pA = if (pA == null) headB else pA.next
pB = if (pB == null) headA else pB.next
}
return pA
}
}
``````

#### PHP

##### Brute force
``````/**
* Definition for a singly-linked list.
* class ListNode {
*     public \$val = 0;
*     public \$next = null;
*     function __construct(\$val) { \$this->val = \$val; }
* }
*/

class Solution
{
/**
* @param ListNode \$headA
* @param ListNode \$headB
* @return ListNode
*/
function getIntersectionNode(\$headA, \$headB)
{
while (\$headA) {
\$pB = \$headB;
while (\$pB) {
if (\$pB === \$headA) {
return \$headA;
}
\$pB = \$pB->next;
}
\$headA = \$headA->next;
}
return null;
}
}
``````
##### Hash table
``````/**
* Definition for a singly-linked list.
* class ListNode {
*     public \$val = 0;
*     public \$next = null;
*     function __construct(\$val) { \$this->val = \$val; }
* }
*/

class Solution
{
/**
* @param ListNode \$headA
* @param ListNode \$headB
* @return ListNode
*/
function getIntersectionNode(\$headA, \$headB)
{
\$nodesInB = [];
while (\$headB) {
\$nodesInB[spl_object_id(\$headB)] = true;
\$headB = \$headB->next;
}

while (\$headA) {
if (isset(\$nodesInB[spl_object_id(\$headA)])) {
return \$headA;
}
\$headA = \$headA->next;
}
return null;
}
}
``````
##### Two pointer
``````/**
* Definition for a singly-linked list.
* class ListNode {
*     public \$val = 0;
*     public \$next = null;
*     function __construct(\$val) { \$this->val = \$val; }
* }
*/

class Solution
{
/**
* @param ListNode \$headA
* @param ListNode \$headB
* @return ListNode
*/
function getIntersectionNode(\$headA, \$headB)
{
\$pA = \$headA;
\$pB = \$headB;
while (\$pA !== \$pB) {
\$pA = \$pA ? \$pA->next : \$headB;
\$pB = \$pB ? \$pB->next : \$headA;
}
return \$pA;
}
}
``````

#### Python

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

class Solution:
def getIntersectionNode(
self, headA: ListNode, headB: ListNode
) -> Optional[ListNode]:
while headA:
pB = headB
while pB:
if headA == pB:
return headA
pB = pB.next
headA = headA.next
return None
``````
##### Hash table
``````# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
def getIntersectionNode(
self, headA: ListNode, headB: ListNode
) -> Optional[ListNode]:
nodes_in_B = set()

while headB:
nodes_in_B.add(headB)
headB = headB.next

while headA:
if headA in nodes_in_B:
return headA
headA = headA.next
return None
``````
##### Two pointer
``````# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
def getIntersectionNode(
self, headA: ListNode, headB: ListNode
) -> Optional[ListNode]:
pA = headA
pB = headB
while pA != pB:
pA = pA.next if pA else headB
pB = pB.next if pB else headA
return pA
``````

#### Rust

``````
``````

#### Swift

##### Brute force
``````/**
* Definition for singly-linked list.
* public class ListNode {
*     public var val: Int
*     public var next: ListNode?
*     public init(_ val: Int) {
*         self.val = val
*         self.next = nil
*     }
* }
*/

class Solution {
func getIntersectionNode(_ headA: ListNode?, _ headB: ListNode?) -> ListNode? {
var a = headA
while a != nil {
var pB = headB
while pB != nil{
if a === pB {
return a
}
pB = pB?.next
}
a = a?.next
}
return nil
}
}
``````
##### Hash table
``````/**
* Definition for singly-linked list.
* public class ListNode {
*     public var val: Int
*     public var next: ListNode?
*     public init(_ val: Int) {
*         self.val = val
*         self.next = nil
*     }
* }
*/

extension ListNode: Equatable, Hashable {
public static func == (lhs: ListNode, rhs: ListNode) -> Bool {
return lhs === rhs
}

public func hash(into hasher: inout Hasher) {
hasher.combine(val + (next?.val ?? 0))
}
}

class Solution {
func getIntersectionNode(_ headA: ListNode?, _ headB: ListNode?) -> ListNode? {
var nodesInB = Set<ListNode>()
var temp = headB

while temp != nil {
nodesInB.insert(temp!)
temp = temp?.next
}

temp = headA
while temp != nil {
if nodesInB.contains(temp!) {
return temp
}
temp = temp?.next
}

return nil
}
}
``````
##### Two pointer
``````/**
* Definition for singly-linked list.
* public class ListNode {
*     public var val: Int
*     public var next: ListNode?
*     public init(_ val: Int) {
*         self.val = val
*         self.next = nil
*     }
* }
*/

class Solution {
func getIntersectionNode(_ headA: ListNode?, _ headB: ListNode?) -> ListNode? {
var pA: ListNode? = headA
var pB: ListNode? = headB
while pA !== pB {
pA = pA == nil ? headB : pA?.next
pB = pB == nil ? headA : pB?.next
}
return pA
}
}
``````

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