L12_iterativePostOrder1StackJava

class Solution {

    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> st1 = new Stack<TreeNode>(); 
        List<Integer> postOrder = new ArrayList<Integer>();
        
        if(root == null) return postOrder; 
        
        TreeNode current = root;  
        while(current != null || !st1.isEmpty()) {
            if(current != null){
                st1.push(current);
                current = current.left;
            }else{
                TreeNode temp = st1.peek().right;
                if (temp == null) {
                    temp = st1.pop();
                    postOrder.add(temp.val);
                    while (!st1.isEmpty() && temp == st1.peek().right) {
                        temp = st1.pop();
                        postOrder.add(temp.val);
                    }
                } else {
                    current = temp;
                }
            }
        }
      
        return postOrder; 
    }
}