如何在Swift中分配和解包可选数组对象？

Question

I'm trying to invert a binary tree in Swift using the iterative approach. Essentially I just keep looping through each node, and putting it into a stack array. This array should be an array of optional nodes. Here is my code:

 func invertTree(_ root: TreeNode?) -> TreeNode? { 
        if root != nil {
            stack.append(root) 
        } else {
            return root
        }

        var stack = [TreeNode?]()
        while stack.count > 0 {
            if stack.last == nil {
                stack.removeLast()
            } else {

                var tempLeft : TreeNode? = stack.last!.left
                stack.last!.left = stack.last!.right
                stack.last!.right = tempLeft

                if stack.last!.left != nil {
                    stack.append(stack.last!.left)
                }

                if stack.last.right != nil {
                    stack.append(stack.last!.right)
                }
            }
        }
        return root
}

I'm getting all sorts of optional errors. In particular, when I set:

var tempLeft : TreeNode? = stack.last!.left

I'm not sure why it says "value of optional type TreeNode? must be unwrapped to refer to member 'left' of unwrapped base type 'TreeNode' in solution.swift"

I don't understand why it's telling me to unwrap the optional, when I'm already force unwrapping it. I don't want to unwrap the .left node since I want to include nils in my stack for the iteration to work.

Any help would be greatly appreciated.

Answer 1

stack.last is a doubly optional TreeNode: TreeNode?? .

You could use optional chaining like so:

if stack.last == nil {
    stack.removeLast()
} else {
    stack.last
    var tempLeft : TreeNode? = stack.last!?.left
    stack.last!?.left = stack.last!?.right
    stack.last!?.right = tempLeft

    if stack.last!?.left != nil {
        stack.append(stack.last!?.left)
    }

    if stack.last??.right != nil {
        stack.append(stack.last!?.right)
    }
}

Or avoid the drama and use pattern matching:

if case let node?? = stack.last {
    let tempLeft = node.left
    node.left = node.right
    node.right = tempLeft

    ...

---

Other than that, it seems that there are other areas of your code that you need to tend to. For example, using stack before its declaration.

---

Alternative implementation

Here is a solution that uses a stack:

class Solution {
    func invertTree(_ root: TreeNode?) -> TreeNode? {

        guard let r = root else {
            return nil
        }

        var stack = [r]

        while let node = stack.last {
            let temp   = node.left
            node.left  = node.right
            node.right = temp

            stack.removeLast()

            if let left  = node.left  { stack.append(left)  }
            if let right = node.right { stack.append(right) }
        }

        return r
    }
}