# 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
``````/**
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/

for pB != nil {
}
pB = pB.Next
}
}
return nil
}
``````
##### Hash table
``````/**
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
var nodesInB = make(map[*ListNode]bool)

}

}
}
return nil
}
``````
##### Two pointer
``````/**
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
for pA != pB {
if pA == nil {
} else {
pA = pA.Next
}
if pB == nil {
} else {
pB = pB.Next
}
}
return pA
}
``````

#### JavaScript

##### Brute force
``````/**
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @return {ListNode}
*/
while (pB) {
}
pB = pB.next;
}
}
return null;
};
``````
##### Hash table
``````/**
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @return {ListNode}
*/
const nodesInB = new Set();

}
}
}
return null;
};
``````
##### Two pointer
``````/**
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @return {ListNode}
*/
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`
* class ListNode(var `val`: Int) {
*     var next: ListNode? = null
* }
*/

class Solution {
while (pA != null) {
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`
* class ListNode(var `val`: Int) {
*     var next: ListNode? = null
* }
*/

class Solution {
var nodesInB = HashSet<ListNode>()
while (node != null) {
node = node.next
}

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`
* class ListNode(var `val`: Int) {
*     var next: ListNode? = null
* }
*/

class Solution {
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
{
/**
* @return ListNode
*/
{
while (\$pB) {
}
\$pB = \$pB->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
{
/**
* @return ListNode
*/
{
\$nodesInB = [];
}

}
}
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
{
/**
* @return ListNode
*/
{
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(
) -> Optional[ListNode]:
while pB:
pB = pB.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(
) -> Optional[ListNode]:
nodes_in_B = set()

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(
) -> Optional[ListNode]:
while pA != pB:
pA = pA.next if pA else headB
pB = pB.next if pB else headA
return pA
``````

#### Rust

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

#### Swift

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

class Solution {
while a != nil {
while pB != nil{
if a === pB {
return a
}
pB = pB?.next
}
a = a?.next
}
return nil
}
}
``````
##### Hash table
``````/**
* 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 {
var nodesInB = Set<ListNode>()

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

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

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

class Solution {
while pA !== pB {
pA = pA == nil ? headB : pA?.next
pB = pB == nil ? headA : pB?.next
}
return pA
}
}
``````