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import java.util.Stack;import java.util.List;import java.util.LinkedList;class Solution {    List
   
     postorderTraversal(TreeNode root) {        List
    
      result = new LinkedList<>();        Stack
     
       stack = new Stack<>();        if (root == null) {            return result;        }        stack.push(root);        while (!stack.isEmpty()) {            TreeNode temp = stack.pop();            result.add(temp.val);            if (temp.left != null) {                stack.push(temp.left);            }            if (temp.right != null) {                stack.push(temp.right);            }        }        return result;    }}
     
    
   

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import java.util.List;import java.util LinkedList;class Solution {    List
   
     result = new LinkedList<>();    public List
    
      postorderTraversal(TreeNode root) {        if (root == null) {            return result;        }        helper(root);        return result;    }    private void helper(TreeNode root) {        if (root == null) {            return;        }        helper(root.left);        helper(root.right);        result.add(root.val);    }}
    
   

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