2022.06.23 (traversal of lc_144,94145\

Method 1 ： recursive

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

Method 2 ： iteration  

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if (root == null) return ans;
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode node = root;
        // Traversal termination conditions ： The current node is empty and the stack is empty 
        while (node != null || !stack.isEmpty()) {
            while (node != null) {
                stack.push(node);
                ans.add(node.val);
                // Find the leftmost node 
                node = node.left;
            }
            node = stack.pop();
            node = node.right;
        }
        return ans;
    }
}