144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

题目含义：用先序方式遍历一个树

方法一：

    private void frontParserTree(TreeNode root,List<Integer> result)
    {
        if (root == null) return;
        result.add(root.val);
        frontParserTree(root.left,result);
        frontParserTree(root.right,result);
    }    
    public List<Integer> preorderTraversal(TreeNode root) {
          List<Integer> result = new ArrayList<>();
        frontParserTree(root,result);
        return result;      
    }

方法二：

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new LinkedList<Integer>();
        Stack<TreeNode> rights = new Stack<TreeNode>();
        while(root != null) {
            list.add(root.val);
            if (root.right != null)  rights.push(root.right);
            root = root.left;
            if (root == null && !rights.isEmpty()) root = rights.pop();
        }
        return list;
    }