inorder.swift

class Solution {
    func inorderTraversal(_ root: TreeNode?) -> [Int] {
        var stack: [(TreeNode, Bool)] = (root != nil) ? [(root!, true)] : []
        var output:[Int] = []
        while let (node, unvisited) = stack.popLast() {
            guard unvisited else {
                output.append(node.val)
                print(node.val)
                continue
            }
            if let right = node.right {
                stack.append((right, true))
            }
            stack.append((node, false))
            if let left = node.left {
                stack.append((left, true))
            }
        }
        return output
    }
}