CODE 141. Binary Tree Preorder Traversal

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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?

public ArrayList<Integer> preorderTraversal(TreeNode root) {
	// IMPORTANT: Please reset any member data you declared, as
	// the same Solution instance will be reused for each test case.
	if (null == root) {
		return new ArrayList<Integer>();
	}
	ArrayList<Integer> result = new ArrayList<Integer>();
	Stack<TreeNode> stack = new Stack<TreeNode>();
	while (!stack.isEmpty() || null != root) {
		if (null != root) {
			result.add(root.val);
			if (null != root.right) {
				stack.push(root);
			}
			root = root.left;
		} else {
			TreeNode node = stack.pop().right;
			result.add(node.val);
			if (null != node.right) {
				stack.push(node);
			}
			root = node.left;
		}
	}
	return result;
}