[LeetCode] Binary Tree Postorder Traversal

public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> nodeStack = new Stack<TreeNode>();
        TreeNode nowNode = root;
        boolean popNode = false;
        
        if (root == null) return result;
        
        while(!(nowNode == root && popNode)) {
            if ( !popNode && nowNode != null) {
                nodeStack.push(nowNode);
                nowNode = nowNode.left;
            } else if ( nowNode == null || (popNode && nowNode != nodeStack.peek().right)) { //left done
                if (nodeStack.peek().right == null) {
                    nowNode = nodeStack.pop();
                    result.add(nowNode.val);
                    popNode = true;
                } else {
                    nowNode = nodeStack.peek().right;
                    popNode = false;
                }                
            } else if (popNode && (nowNode == nodeStack.peek().right)) { //right done
                nowNode = nodeStack.pop();
                result.add(nowNode.val);
                popNode = true;                
            }
        }
        
        return result;
    }
}