https://leetcode.com/problems/binary-tree-postorder-traversal/ Swift Solution Loop approach

BinaryTreePostorderTraversal.swift

func postorderTraversal(root:TreeNode) -> [Int]{
    var arr = [Int]()
    var nodes = [TreeNode]()
    var middleNodes = [TreeNode]()
    var current :TreeNode? = root
    var lastMiddleNode : TreeNode?
    while current != nil{
        let cur = current!
        if cur.left == nil && cur.right == nil{
            // 没有子节点,访问此节点
            arr.append(cur.val)
            if cur === root{
                // 已经是根节点了
                break
            }
            // 往回走
            current = nodes.removeLast()
            lastMiddleNode = middleNodes.last
            while current === lastMiddleNode{
                arr.append(current!.val)
                middleNodes.removeLast() //补之前的一次删除
                if nodes.isEmpty{
                    current = nil
                    break
                }
                
                current = nodes.removeLast()
                lastMiddleNode = middleNodes.last
                
            }
        }else{
            //左边或者右边还有节点.先把中间的节点保存下来.以备后用
            nodes.append(current!)
            middleNodes.append(current!)
            if let left = current?.left{
                // 准备继续向左走,把当前节点右子节点保存下来
                if let right = current?.right{
                    nodes.append(right)
                }
                // 向左走
                current = left
            }else{
                // 向右走 这种情况下左右必须有.因为else中必须一个有节点
                current = current?.right
            }
        }
    }
    return arr
}