Solution.java

// 递归版本
/*
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        preorderTraversal(root, list);
        return list;
    }
    
    private void preorderTraversal(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        list.add(root.val);
        preorderTraversal(root.left, list);
        preorderTraversal(root.right, list);
    }
}

*/

// 非递归版本
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (root != null) {
            list.add(root.val);
            if (root.right != null) {
                stack.push(root.right);
            }
            if (root.left != null) {
                stack.push(root.left);
            }
            if (!stack.isEmpty()) {
                root = stack.pop();
            } else {
                break;
            }
        }
        
        return list;
    }
}