二叉树的前序遍历

二叉树前序遍历，分为递归方法和非递归方法：

递归：

private static List<Integer> preList = new ArrayList<>();
public static List<Integer> preorderTraversalRec(TreeNode root){
    if (null == root){
        return preList;
    }
    preorder(root);
    return preList;
}
private static void preorder(TreeNode root){
    if (null == root){
        return;
    }
    preList.add(root.val);
    preorder(root.left);
    preorder(root.right);
}

非递归：

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