find the variance of all nodes in a linked list in swift

main.swift
class Node {
    var value: Double
    var next: Node?

    init(_ value: Double, _ next: Node? = nil) {
        self.value = value
        self.next = next
    }
}

func variance(of list: Node?) -> Double {
    var mean = 0.0
    var count = 0
    
    // Traverse the linked list and find the mean
    var current = list
    while current != nil {
        mean += current!.value
        count += 1
        current = current!.next
    }
    mean /= Double(count)
    
    // Traverse the linked list again and accumulate the squared difference
    var variance = 0.0
    current = list
    while current != nil {
        let diff = current!.value - mean
        variance += diff * diff
        current = current!.next
    }
    
    // Divide accumulated value by the total number of nodes to get variance
    variance /= Double(count)
    return variance
}
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main.swift
let list = Node(2, Node(4, Node(6)))
let result = variance(of: list) // returns 2.6666666666666665
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