Three way of post order traversal

postOrderTraversal.java

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

    public List<Integer> postorderTraversalOneStack(TreeNode root) {

        List<Integer> l = new LinkedList<Integer>();
        Stack<TreeNode> s = new Stack<>();

        TreeNode traverse = root;
        TreeNode prev = null;
        while(traverse != null || !s.isEmpty()) {
            
            while(traverse != null) {
                s.push(traverse);
                traverse = traverse.left;
            }
            
            if(s.peek().right != null && prev != s.peek().right) {
                traverse = s.peek().right;
            } else {
                l.add(s.peek().val);
                prev = s.pop();
            }
        }
        
        return l;
    }
    
    public List<Integer> postOrderRecursive(List<Integer> l, TreeNode node) {
        if(node == null) {
            return l;
        }
        
        postOrderRecursive(l, node.left);
        postOrderRecursive(l, node.right);
        
        l.add(node.val);
        return l;
    }
}