[Leetcode] Binary tree travelsal (preorder, inorder, postorder)

一、前序

public List<Node> preOrder(Node root){
    List<Node> res = new LinkedList<Node>();
    Stack<Node> stack = new Stack<Node>();
    stack.push(root);
    while(root!=null||!stack.isEmpty()){
        while(root){
            res.add(root);
            stack.push(root);
            root=root.left;
        }
        Node tmp = stack.pop();
        root= tmp.right;
    }
    return res;
}

二、中序

public List<Node> inOrder(Node root){
    List<Node> res = new LinkedList<Node>();
    Stack<Node> stack = new Stack<Node>();
    stack.push(root);
    while(root!=null||!stack.isEmpty()){
        while(root){
            stack.push(root);
            root= root.left;
        }
        Node tmp = stack.pop();
        res.add(tmp);
        root=tmp.right;
    }
}

三、后序

public List<Node> postOrder(Node root){
    List<Node> res = new LinkedList<Node>();
    Stack<Node> stack = new Stack<Node>();
    stack.push(root);
    Node head = root;
    while(!stack.isEmpty()){
        Node current =stack.peek();//do not know if should pop()
        if(current.right==head||current.left==head||(current.left==null||current.right==null)){
            stack.pop();
            res.add(current);
            head=current;
        }else{
            if(current.right!=null) stack.push(current.right);
            if(current.left!=null) stack.push(current.left);
        }
    }
}