leetcode-160-intersection-of-two-linked-lists.swift

/**

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https://leetcode.com/problems/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:


begin to intersect at node c1.

 

Example 1:


Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,0,1,8,4,5], skipA = 2, skipB = 3
Output: Reference of the node with value = 8
Input 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 [5,0,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.

*/

/**
Big O Annotation
Time complexity O(n) where n is the combined number of elements in headA and headB.
Space complexity O(n) where n is the combined number of elements in headA and headB.
*/
func getIntersectionNode(_ headA: ListNode?, _ headB: ListNode?) -> ListNode? {
        
    var valueListA: Set<ListNode> = []
    var inserSection: ListNode?
    
    // Traverse A into the Set
    headA?.traverse { valueListA.insert($0) }
    
    // Traverse B and get the intersection
    headB?.traverse { node in 

        /**
        Make sure to not overwrite with later nodes
        as that would not be the intersection but
        common nodes.

        We could optimize this by interrupting the close calls
        and directly returning.
        */
        if valueListA.contains(node) && inserSection == nil {
            inserSection = node
        }
    }
    
    return inserSection
}

extension ListNode {

    // Function to traverse the list
    func traverse(visit: (ListNode) -> Void) {
        visit(self)
        next?.traverse(visit: visit)
    }
}

/**
Conform ListNode to Hashable to use in a Set
and to take advantage of constant contains operation.

Using ObjectIdentifier to use the memory reference for equality.
*/
extension ListNode: Hashable {

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

    public func hash(into hasher: inout Hasher) {
        hasher.combine(val)
    }
}