Binary Tree Traversal - two styles of solutions

通过查询资料，主要有两种风格的iterative solution for binary tree traversal

For example, for the preorder traversal, there are two kinds of iterative solutions. Pay attention to the condition of the while loop:

https://leetcode.com/problems/binary-tree-preorder-traversal/

1.指针遍历同时压栈根节点法 while(root!=null||!s.isEmpty()):

http://blog.csdn.net/linhuanmars/article/details/21428647

List<Integer> res = new ArrayList<Integer>();
while(root!=null||!s.isEmpty()){
            if(root!=null){
                res.add(root.val);
                s.push(root);
                root = root.left;
            }
            else{
                TreeNode n = s.pop();
                root = n.right;
                
            }
        }
        return res;

2. 先压栈子节点后弹栈遍历法while(!s.isEmpty())：

https://algorithm.yuanbin.me/zh-hans/binary_tree/binary_tree_preorder_traversal.html

List<Integer> res = new ArrayList<Integer>();        
        if(root==null){
            return res;
        }
        LinkedList<TreeNode> s = new LinkedList<TreeNode>();
        s.push(root);
        while(!s.isEmpty()){
            TreeNode n = s.pop();
            res.add(n.val);
            if(n.right!=null){
                s.push(n.right);
            }
            if(n.left!=null){
                s.push(n.left);
            }
            
        }
        return res;

原本我以为第二种方法好一些，但后来发现inorder用第二种方法很难作。因此为了统一记忆方便，仍是都选用第一种只压栈根节点作法吧！