/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
递归
class Solution {
    List<Integer> lists = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if(root == null) return lists;
        lists.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return lists;
    }
}

非递归 利用栈
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> lists = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        while(curr != null||!stack.isEmpty()){
            while(curr != null){
                stack.push(curr);
                lists.add(curr.val);
                curr = curr.left;
            }
            curr = stack.pop(); //弹出栈顶元素
            curr = curr.right;
            // curr = stack.pop().right;
        }
        return lists;
    }
}