[1,null,2,3] 1 \ 2 / 3

 [3,2,1]

Solution1 – Non-Recursive

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        
        List<Integer> list = new ArrayList<Integer>();
        if(root == null) return list;
        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        
        s1.push(root);
        while(!s1.isEmpty()){
            TreeNode node = s1.pop();
            s2.push(node);
            if(node.left!=null){
                s1.push(node.left);
            }
            if(node.right!=null){
                s1.push(node.right);
            }
        }
        
        while(!s2.isEmpty()){
            TreeNode node = s2.pop();
            list.add(node.val);
        }
        
        return list;
    }
}

Solution2 – Recursive