Iterative unified writing method of binary tree

Put the accessed node on the stack , Put the node to be processed in the stack, but mark it .

How to mark , After the nodes to be processed are put on the stack , Then put a null pointer as a marker .

Before the order ：

var preorderTraversal = function(root) {
    let s=[root]
    let arr=[]
    if(root===null){
        return arr
    }
    while(s.length){
        // Take out the head node 
        let node=s.pop()
        // If the current node is a marked node 
        if(node==null){
            // Put the next node （ root ） Save to array 
            arr.push(s.pop().val)
            continue
        }
        // Right 
        node.right&&s.push(node.right)
        // Left 
        node.left&&s.push(node.left)
        // root 
        s.push(node)
        // Mark 
        s.push(null)
    }
    return arr
};

In the following order ：

var postorderTraversal = function(root) {
    let s=[root]
    let arr=[]
    if(root===null){
        return arr
    }
    while(s.length){
        let node=s.pop()
        if(node===null){
            arr.push(s.pop().val)
            continue
        }
        // root 
        s.push(node)
        // Mark 
        s.push(null)
        // Right 
        node.right&&s.push(node.right)
        // Left 
        node.left&&s.push(node.left)

    }
    return arr
};

Middle preface ：

var inorderTraversal = function(root) {
    let s=[root]
    let arr=[]
    if(root===null){
        return arr
    }
    while(s.length){
        let node=s.pop()
        if(node===null){
            arr.push(s.pop().val)
            continue
        }
        // Right 
        node.right&&s.push(node.right)
        // root 
        s.push(node)
        s.push(null)
        // Left 
        node.left&&s.push(node.left)
    }
    return arr
};