145. Binary Tree Postorder Traversal!

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> result = new LinkedList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        if(root==null) return result;
        stack.push(root);
        while(stack.size()>0){
            TreeNode current = stack.pop();
            // this is the parent node, add it from the beginning
            result.addFirst(current.val);
            // add left first before right node
            if(current.left!=null)stack.push(current.left);
            if(current.right!=null)stack.push(current.right);
        }
        return result;
    }
}