Binary Tree Preorder Traversal

题目

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?

方法

private Stack<TreeNode> stack = new Stack<TreeNode>();    public ArrayList<Integer> preorderTraversal(TreeNode root) {ArrayList<Integer> al = new ArrayList<Integer>();if (root != null) {    stack.push(root);    while (stack.size() != 0) {    TreeNode node = stack.pop();;    al.add(node.val);        if (node.right != null) {    stack.push(node.right);    }    if (node.left != null) {    stack.push(node.left);    }    }} return al;    }