二叉树前序遍历

/*
 * 前序遍历二叉树
 */
public List<Integer> preorderTraversal(TreeNode1 root) {
    List<Integer> list = new LinkedList<>();
    if(root == null)
        return null;
    Stack<TreeNode1> stack = new Stack<TreeNode1>();
    stack.add(root);
    while(!stack.isEmpty()) {
    	TreeNode1 curNode = stack.pop();
    	list.add(curNode.val);
    	if(curNode.right != null) {
    		stack.add(curNode.right);
    	}
    	if(curNode.left != null) {
    		stack.add(curNode.left);
    	}
    }
    return list;
}