(!stack.isEmpty()) { TreeNode cur = stack.pop(); ans.addFirst(cur.val); if (cur.left != null) { stack.push(cur.left); } if (cur.right != null) { stack.push(cur.right); } } return ans; }

Method 4 - One Stack with Tree Modification

Code

[{"language":"Java","code":"public List < Integer > postorderTraversal(TreeNode root) {\n    List < Integer > res = new ArrayList < > ();\n\n    if (root == null) {\n        return res;\n    }\n\n    Stack < TreeNode > stack = new Stack < TreeNode > ();\n    stack.push(root);\n\n    while (!stack.isEmpty()) {\n        TreeNode temp = stack.peek();\n        if (temp.left == null && temp.right == null) {\n            TreeNode pop = stack.pop();\n            res.add(pop.val);\n        } else {\n            if (temp.right != null) {\n                stack.push(temp.right);\n                temp.right = null;\n            }\n\n            if (temp.left != null) {\n                stack.push(temp.left);\n                temp.left = null;\n            }\n        }\n    }\n\n    return res;\n}","lang":"java","html":"<pre class=\"shiki shiki-themes github-light github-dark\" style=\"background-color:#fff;--shiki-dark-bg:#24292e;color:#24292e;--shiki-dark:#e1e4e8\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">public</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> List </span><span style=\

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