Binary Tree Postorder Traversal

recursion programming

public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        traversal(result, root);
        return result;
    }
    private void traversal(ArrayList<Integer> result, TreeNode node)
    {
        if(node == null)
            return; 
        traversal(result, node.left);
        traversal(result, node.right);
        result.add(node.val);
    }
}

public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(root == null){
            return result;
        }
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = null, pre = null;
        stack.push(root);
        
        while(!stack.empty()){
            cur = stack.peek();
            if((cur.left == null && cur.right == null) ||
            ((pre != null) && (cur.left == pre || cur.right == pre))){
                result.add(cur.val);
                pre = cur;
                stack.pop();
            } else {
                if(cur.right != null){
                    stack.push(cur.right);
                }
                if(cur.left != null){
                    stack.push(cur.left);
                }
            }
            
        }
        
        return result;
    }